Aviation words and our own terms, in plain language

Glossary

Each entry says where it comes from. MANAIR: Environment and Climate Change Canada's Manual of Standards and Procedures for Aviation Forecasts, 8th edition, amendment 15, July 2026, by PDF page, reworded here. This site: how MeteoFrontier uses the term. General usage: written from general aviation and weather knowledge and not yet checked against MANOBS or another manual. This page explains words. It isn't a regulation or a training source.

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Reports and forecasts

METAR: The routine weather report from an airport, normally issued once an hour. It's what was observed, not a forecast.General usage
SPECI: A special weather report issued between METARs because conditions changed significantly, such as a ceiling or visibility crossing an important value.General usage
TAFAerodrome forecast: An issued forecast for one airport, in a fixed coded format. In Canada it's normally updated every six hours. TAF-style text on this site is experimental guidance from MeteoFrontier, not an issued TAF.MANAIR ch. 2 · This site
TEMPO: A temporary change in a TAF. Each spell should last under an hour, and together they cover no more than half of the stated period. Only what changes is listed after the word.MANAIR ch. 2, 2.6.15
PROBPROB30, PROB40: A 30 % or 40 % chance of the stated conditions, used for things that could affect aircraft, such as thunderstorms, freezing precipitation, low-level wind shear and important ceiling or visibility values. Below 30 % isn't worth the group. At 50 % or more, TEMPO, BECMG or FM is used instead.MANAIR ch. 2, 2.6.14
FMFrom: A rapid and lasting change in a TAF at the stated time. FM271100 means from 1100Z on the 27th.MANAIR ch. 2, 2.6.16
BECMGBecoming: A gradual but lasting change in a TAF over the stated period.MANAIR ch. 2, 2.6.16
GFAGraphic area forecast: Six charts for one region, issued four times a day: clouds and weather, plus icing, turbulence and freezing level, each at T+0, T+6 and T+12.MANAIR p. 57
SIGMET: A warning of hazardous weather, such as frequent thunderstorms, severe turbulence, severe icing or volcanic ash.MANAIR p. 89, 179
AIRMET: A warning of weather that wasn't in the GFA, such as widespread low cloud or visibility, moderate icing or moderate turbulence.MANAIR p. 116, 209
PIREPPilot report: Weather reported by a pilot in flight, such as turbulence, icing or cloud tops.General usage
AUTOAutomated report: A METAR made by an automatic station with no observer. Some things, such as cloud above the sensor's reach or weather in the vicinity, may be missing or simplified.General usage
RMKRemarks: The end part of a METAR with extra detail. In Canada it lists cloud types and their oktas layer by layer, for example SC6AC2.General usage
CAVOK: Ceiling and visibility OK: visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude, no CB or TCU and no significant weather. Used internationally; Canadian TAFs and METARs do not use it.General usage
NSWNo significant weather: Used in a TAF change group to say the weather in the previous group has ended.General usage
AMD, COR: An amended TAF (a new version because the old one no longer fit), and a corrected report (an error fixed).General usage
Validity period: The hours a TAF covers, such as 1112/1212: from 1200Z on the 11th to 1200Z on the 12th.General usage
RVRRunway visual range: How far a pilot on the runway centreline can see the runway lights or markings, measured by instruments. Reported in feet in Canada when visibility is low.General usage
Altimeter settingA2992: The pressure, in inches of mercury in Canada, that pilots set so the altimeter reads airport elevation on the ground.General usage
MSLPMean sea-level pressure: Surface pressure adjusted to sea level so places at different heights can be compared on one map. Lines of equal MSLP are isobars.General usage
Wind direction: Always the direction the wind blows from. 27015KT is from the west at 15 knots. METAR and TAF directions are true; ATIS and tower winds are magnetic.General usage
VRBVariable: Wind direction that keeps changing, normally used when the wind is light.General usage
Gust: A brief peak in wind speed above the mean. Reported after G, as in 20G30KT. On this site's TAF-style text, gusts are not coded when the mean wind is under 10 KT.General usage · This site
SquallSQ: A sudden increase in wind that lasts at least a minute, longer than a gust, often with a line of storms.General usage
ATIS: A recorded broadcast at an airport with the current weather, runway in use and other information, updated at least hourly.General usage
NOTAM: A notice to airmen about changes such as closed runways or unserviceable navigation aids. Not a weather product.General usage
Upper winds (FD): Forecast winds and temperatures at set heights above a station, used for flight planning.General usage
Intensity: −, no sign, +: The prefix on a weather code. A minus sign is light, no sign is moderate and a plus sign is heavy, as in -SN, SN, +SN. For precipitation it is judged mainly by rate, and in snow also by how much it cuts visibility.Why it matters. Heavy snow alone can drop visibility to IFR or worse, so +SN is a strong hint about the visibility even before you read it.See RA, SN, DZ, PL, MI, BC, PR, DR, BL, SH, TS, FZ · Learn: Reading a METAR and a TAFGeneral usage
MI, BC, PR, DR, BL, SH, TS, FZWeather descriptors: Two-letter words placed before a weather code to say how it behaves: shallow (MI), patches (BC), partial (PR), drifting below eye level (DR), blowing to eye level or higher (BL), showers (SH), thunderstorm (TS) and freezing (FZ).Why it matters. The descriptor changes the meaning a lot: BLSN can give near-zero visibility under a clear sky, while DRSN barely affects it.See RA, SN, DZ, PL, BLSN, DRSN, MIFG, BCFG, PRFG, FZRA, FZDZ · Learn: Reading a METAR and a TAFGeneral usage
VCVicinity: Placed before a weather code to say it is near the airport but not at it, as in VCSH or VCFG. MANOBS and MANAIR define the distance.Why it matters. A shower or fog bank in the vicinity can move over the field within minutes, so treat VC as a warning, not as good news.See TS, VCTS, RA, SN, DZ, PLGeneral usage
GR, GSHail, small hail or snow pellets: GR is hail: hard lumps of ice from a thunderstorm. GS covers small hail and snow pellets, soft or semi-hard grains under about 5 mm.Why it matters. Hail at the surface means a strong updraft nearby, and much larger hail may be falling under the core aloft.See TS, VCTS, CB, TCU, ACC, CU, Hail size forecastGeneral usage
SG, ICSnow grains, ice crystals: Snow grains are very small, flat white grains, the frozen cousin of drizzle. Ice crystals (diamond dust) are tiny ice needles or plates drifting in very cold, often clear air, and can sparkle in sunlight.Why it matters. Both point to a shallow, cold, moist layer. Ice crystals in arctic air can cut visibility at northern airports even with no cloud reported.See Ice fog, RA, SN, DZ, PLGeneral usage
UPUnknown precipitation: Used by automated stations when the sensor detects precipitation but can't tell what type it is.Why it matters. UP often shows up in mixed or changing precipitation, exactly when the type matters most, so check nearby reports and temperatures.See AUTO, Present weather sensorGeneral usage
FC, +FCFunnel cloud, tornado or waterspout: FC is a funnel cloud not reaching the ground. +FC is a tornado or waterspout: a funnel in contact with the ground or water.Why it matters. Any FC in a report means a violent storm close by. Stay well clear of the parent thunderstorm.See TS, VCTS, CB, TCU, ACC, CU, TornadoGeneral usage
DU, SA, DS, SS, PODust and sand: Widespread dust (DU), sand (SA), dust storm (DS), sandstorm (SS) and dust or sand whirls (PO, dust devils).Why it matters. Rare at most Canadian airports, but blowing dust over dry Prairie fields in spring can cut visibility sharply ahead of a cold front.See FG, BR, HZ, FU, VisibilityGeneral usage
VAVolcanic ash: Volcanic ash in the air. Aircraft avoid it because ash can stop jet engines, scour windscreens and block sensors.Why it matters. Ash from Alaskan or Kamchatkan volcanoes can drift over western and northern Canada; it is handled by SIGMETs and volcanic ash advisories.See VAAC, SIGMETGeneral usage
RERecent weather: A group near the end of a METAR for weather that happened since the last routine report but has stopped, such as REFZRA for recent freezing rain.Why it matters. Recent freezing rain or snow means the aircraft, runway and taxiways may already be contaminated even if nothing is falling now.See FZRA, FZDZ, METARGeneral usage
CLR: Used by automated stations when no cloud is detected below the instrument's height limit. It does not mean the whole sky is clear: cloud higher up can't be seen by the sensor.Why it matters. CLR at an AUTO site can hide a high deck, or a fog bank that hasn't drifted over the sensor yet.See SKC, AUTO, CeilometerGeneral usage
SLPSea-level pressure remark: A Canadian METAR remark giving the pressure reduced to sea level, in hPa, by its last three digits with a decimal before the last one. SLP136 is 1013.6 hPa, SLP987 is 998.7 hPa.Why it matters. It's the number that goes on weather maps, so it ties the airport to the surface analysis. The altimeter setting is the number that goes in the aircraft.See MSLP, Altimeter setting · Learn: Reading a METAR and a TAFGeneral usage
PRESRR, PRESFR: Remarks for pressure rising rapidly or falling rapidly.Why it matters. A rapid fall often means a deepening low or an approaching front; a rapid rise behind a cold front often comes with strong, gusty wind.See SLP, Low, highGeneral usage
NIL: Used in place of a missing report.Why it matters. A missing METAR at the time you need it is a planning problem in itself; nearby stations and the TAF become your only guide.General usage
CNLCancelled: Marks a cancelled forecast, for example a TAF that won't be continued.See AMD, CORGeneral usage
RMK NXT FCST BY: The remark at the end of a Canadian TAF giving the time the next routine TAF is due, such as RMK NXT FCST BY 00Z.Why it matters. If you're planning past that time, plan to check again: the next issue may tell a different story.See TAF, Validity period · Learn: Reading a METAR and a TAFGeneral usage
WS groupWind shear in a TAF: A TAF group for non-convective low-level wind shear, giving the height of the shear layer in hundreds of feet and the wind at that height: WS015/24045KT means shear up to 1,500 ft, with the wind there from 240° at 45 KT.Why it matters. It most often appears overnight with a strong low-level jet above a calm, cold surface layer: a smooth surface wind can turn into a big airspeed change on approach.See LLWS, Nocturnal low-level jet, InversionGeneral usage
Prevailing conditions: In a TAF, the conditions in the main body or the latest FM or BECMG group, which hold except during TEMPO spells or PROB chances.Why it matters. Planning to the prevailing line alone can miss a TEMPO drop below your limits. Read every group that covers your time.See TEMPO, PROB, FM · Learn: Reading a METAR and a TAFGeneral usage
Amendment criteria: The thresholds, such as key ceiling and visibility values or the start of freezing precipitation or thunderstorms, that oblige a forecaster to amend a TAF when the forecast and reality no longer match. MANAIR lists them for Canada.Why it matters. They explain why a TAF is amended for a change from 3 to 2 SM but not from 6 to 4 SM.See AMD, COR, TAFGeneral usage
GFA clouds and weather: The GFA chart showing cloud layers, bases and tops, visibility, weather and fronts for one region. It comes in three panels: now, +6 h and +12 h, plus an outlook line.Why it matters. It's the big-picture view for VFR planning between airports where there's no TAF.See GFA, GFA icing, turbulence and freezing level, ISOL, OCNL, FRQ, LCA, PTCHY, XTNSVGeneral usage
GFA icing, turbulence and freezing level: The companion GFA chart showing areas of icing and turbulence with their intensity and heights, plus freezing-level contours, for the same times as the clouds and weather chart.Why it matters. It's the main planning tool for icing and turbulence on routes without SIGMETs or AIRMETs in force.See GFA, Icing: RIME, MX, CLR, Turbulence: CAT, MECH, MTW, FZLVLGeneral usage
SIGWX chartSignificant weather prog: A forecast chart of significant weather for upper levels: jet streams, turbulence, thunderstorm areas, the tropopause and volcanic ash, made for high-level flights.Why it matters. Useful for a quick look at where clear-air turbulence and jet cores are expected along a long route.See Jet stream, Turbulence: CAT, MECH, MTW, TropopauseGeneral usage
Prog chart: Short for prognostic chart: any forecast map showing where fronts, highs, lows and weather are expected at a future time.See Front, Low, highGeneral usage
UA, UUARoutine and urgent PIREP: UA marks a routine pilot report, UUA an urgent one, used for hazards such as severe turbulence or icing, low-level wind shear or volcanic ash.Why it matters. A fresh UUA near your route outranks any forecast: someone has just met the hazard.See PIREPGeneral usage
VAACVolcanic Ash Advisory Centre: One of nine centres worldwide that track volcanic ash and issue advisories and charts. Montréal's VAAC, run by ECCC, covers Canada.Why it matters. Ash SIGMETs are based on VAAC advisories.See VA, SIGMETGeneral usage
FICFlight Information Centre: NAV CANADA's briefing service, where flight service specialists give weather briefings, take flight plans and pass on PIREPs.Why it matters. A human briefer can connect the TAFs, GFA and PIREPs for your route, and pass your PIREP on to others.See PIREP, NOTAMGeneral usage
Weather briefing: Going through every product that applies to a flight: the big picture (GFA, analysis), then hazards (SIGMET, AIRMET, PIREP), then the departure, en-route and destination reports and forecasts, and NOTAMs.Why it matters. Starting with the big picture makes the individual reports make sense, and stops a good-looking METAR from hiding a bad trend.See GFA, SIGMET, PIREP, NOTAM · On the site: Route briefingGeneral usage
Trend: The direction conditions are moving over the last few reports: ceiling lowering, visibility improving, wind backing, pressure falling.Why it matters. Two METARs an hour apart tell you more than one. A ceiling of 1,200 ft that was 3,000 ft two hours ago is a different story from one that was 600 ft.See METAR, SPECI, PersistenceGeneral usage
ECCCEnvironment and Climate Change Canada: The federal department whose Meteorological Service of Canada makes Canada's weather forecasts and warnings, including the aviation forecasts (TAFs, GFAs, SIGMETs and AIRMETs) done for NAV CANADA.See MSC, CMAC, NAV CANADAGeneral usage
MSCMeteorological Service of Canada: The weather service inside ECCC: observing networks, the Canadian models (GDPS, RDPS, HRDPS), forecasts and warnings.See ECCC, GDPS, HRDPSGeneral usage
CMACCanadian Meteorological Aviation Centre: ECCC's aviation forecast offices, CMAC-East in Montréal and CMAC-West in Edmonton, where forecasters write Canadian TAFs, GFAs, SIGMETs and AIRMETs.See ECCC, TAF, GFAGeneral usage
NAV CANADA: The private, non-profit company that runs Canada's civil air navigation system: air traffic control, flight information centres and the aviation weather observing and briefing services, including publishing TAFs, METARs and GFAs.See FIC, AWOS, ECCCGeneral usage
MANOBS: ECCC's Manual of Surface Weather Observation Standards: the Canadian rulebook for how weather is observed and coded in METARs and SPECIs.See METAR, MANAIRGeneral usage
MANAIR: ECCC's Manual of Standards and Procedures for Aviation Forecasts: the Canadian rulebook for TAFs, GFAs, SIGMETs, AIRMETs and upper wind forecasts. Many glossary entries cite it.See TAF, GFA, MANOBSGeneral usage
TC AIMTransport Canada Aeronautical Information Manual: Transport Canada's reference manual for pilots. Its MET section explains the Canadian weather services and products and how pilots are expected to use them.See CARs, MANAIRGeneral usage
CARsCanadian Aviation Regulations: The regulations that govern flying in Canada, including VFR weather minima, alternate requirements and fuel rules. Weather pages explain conditions; the CARs decide what is allowed.See VFR weather minima, TC AIMGeneral usage
ICAO, WMO: The International Civil Aviation Organization and the World Meteorological Organization, UN agencies that set the international standards for aviation weather codes such as METAR and TAF. Countries, Canada included, add their own national practices.See METAR, TAFGeneral usage
NOAA: The US National Oceanic and Atmospheric Administration: the US National Weather Service, the RAP, HRRR, RRFS and GFS models, and the GOES satellites used on this site.See RAP, RRFS, GOESGeneral usage
Always-amend weather: Weather that requires a TAF amendment whenever it starts, stops or is newly expected, regardless of visibility or duration: thunderstorms, hail, freezing rain, freezing drizzle, freezing fog, ice pellets, snow grains, squalls, funnel clouds and dust or sand storms. Other precipitation and obscurations count only when the visibility is 6 SM or less.See Amendment criteria, FZRA, FZDZ, Ice pellets or freezing rain · Learn: Writing and amending TAFs: the desk viewGeneral usage
Wind amendment criteria: The wind changes that call for a TAF amendment: the mean speed doubles or halves with at least one of the two values above 15 KT, the speeds differ by 20 KT or more, or the direction is off by 45° or more while the observed wind is above 15 KT.Why it matters. A 90° direction error at 10 KT is a quality issue; the same error at 20 KT changes the runway in use.See Amendment criteria, Gust, Wind shift · Learn: Writing and amending TAFs: the desk viewGeneral usage
Prevailing conditions (TAF): The conditions a TAF expects to dominate a period: the initial group and each FM group. TEMPO describes short departures from them, PROB a chance of something else, BECMG a gradual transition between two prevailing states.Why it matters. Readers plan on the prevailing conditions; the change groups tell them what else to carry fuel for.See FM, TEMPO, PROB, BECMG · Learn: Writing and amending TAFs: the desk viewGeneral usage
FM with minutes: An FM group timed to the minute, such as FM141730, used when the timing of a frontal passage or a fog clearance is known well enough. A TEMPO or PROB group next to it is written to the whole hour and may overlap the FM time by less than an hour.See FM, FROPA · Learn: Writing and amending TAFs: the desk viewGeneral usage
Thunderstorm constructs (TAF): The ladder of ways a TAF can express thunderstorm risk: thunderstorms in the prevailing group, TEMPO, PROB40, PROB30, and TEMPO VCTS, each tied to a band of probability at the aerodrome. Only CB may be forecast as the cloud type, and every thunderstorm group has a CB in force.See TS, VCTS, VC, PROB, TEMPO, CB, TCU, ACC, CU · Learn: Convective initiation and severe parameters for aviationGeneral usage
Runway heading (true and magnetic): A runway's number is its magnetic heading rounded to tens of degrees (runway 26 ≈ 260° magnetic). Its true heading, measured from geographic north, differs by the local magnetic declination, which in Canada ranges from about 15° east on the BC coast to more than 15° west in the east and much more in the Arctic.Why it matters. METAR and TAF winds are true, while runway numbers and tower winds are magnetic. Crosswind sums must use one or the other consistently; this site uses true headings from the runway threshold positions.See Crosswind, Headwind and tailwind component, METAR · On the site: Airport intelligenceGeneral usage; Transport Canada AIM

Observations and instruments

AWOSAutomated weather observation system: An automatic weather station at an airport that measures wind, temperature, dew point, pressure, visibility, cloud and precipitation, and writes METARs marked AUTO.Why it matters. AUTO reports are useful but limited: cloud above the ceilometer's reach, distant weather and some precipitation types can be missed.See AUTO, Ceilometer, Visibility sensorGeneral usage
LWISLimited weather information system: A reduced automatic station at some Canadian airports that reports only a few elements, such as wind, temperature, dew point and altimeter setting, without the full set an AWOS gives.Why it matters. An LWIS report can look complete at a glance, but it may tell you nothing about cloud or visibility.See AWOS, AUTOGeneral usage
Human observer: A trained person who takes the observation, often helped by instruments. Observers can see the whole sky, distant weather, cloud types and things sensors miss.Why it matters. Cloud types in the remarks, distant CB, virga and the true extent of fog patches usually come from a human eye.See METAR, RMK, AWOSGeneral usage
Ceilometer: A laser that points straight up and times the echoes from cloud droplets to measure cloud base height. It sees only what passes over the airport.Why it matters. It is accurate overhead but blind to the rest of the sky, and its reach is limited, so high cloud may not be reported at automatic stations.See Cloud base, AWOS, CLRGeneral usage
Visibility sensor: An instrument that measures how much light a small volume of air scatters and converts it to a visibility. It samples a few cubic metres near the sensor.Why it matters. In patchy fog or snow showers, one small sample can read much better or worse than the visibility across the airport.See Visibility, AWOS, RVRGeneral usage
Present weather sensor: An automatic sensor that detects precipitation and tries to classify it, for example by the size and fall speed of the particles.Why it matters. Freezing drizzle, ice pellets and mixed precipitation are the hardest types for it. The result can be UP or a wrong type.See UP, AWOS, FZRA, FZDZGeneral usage
Transmissometer: An instrument beside a runway that measures how much light is lost across a known path. RVR is computed from it, together with the runway light setting and background brightness.See RVR, Visibility sensorGeneral usage
Anemometer, wind vane: The instruments that measure wind speed and direction, normally on a mast 10 m above the ground. Reported wind is an average over a short period, with gusts taken from the peaks.Why it matters. Obstacles, buildings and a sheltered mast can make one airport's wind unrepresentative.See Gust, Wind direction, Mean windGeneral usage
Mean wind: The average wind over a short period, a couple of minutes in a METAR. Gusts are reported separately when the peaks rise well above that mean.Why it matters. Crosswind limits are usually applied to both the mean and the gust; the gust is what you'll feel in the flare.See Gust, Anemometer, wind vane, CrosswindGeneral usage
Instrument screen: The ventilated white shelter, about 1.5 to 2 m above grass, that holds the thermometer and humidity sensor so they read the air and not sunshine or the ground.Why it matters. Air at the screen can be several degrees warmer than at the runway surface on a clear, calm night, which is how frost or fog forms while the METAR shows a spread of a few degrees.See Dew point, Radiative coolingGeneral usage
Barometer: The instrument that measures air pressure. Station pressure is then corrected to give the altimeter setting and the sea-level pressure.See Altimeter setting, SLP, QNHGeneral usage
RadiosondeWeather balloon: A small instrument package carried up by a balloon, measuring temperature, humidity and pressure, with wind from its GPS track. Launched at about 00Z and 12Z from a network of stations.Why it matters. Balloon soundings are the ground truth for inversions, freezing levels and instability. Models are checked against them.See Sounding, Tephigram, 00Z and 12Z · On the site: Observed soundings · Learn: Reading a soundingGeneral usage
00Z and 12ZBalloon launch times: The two main daily times for balloon soundings worldwide. Balloons go up a little before the hour so they're in mid-flight at it. In western Canada, 00Z is late afternoon or early evening and 12Z is early morning, local time.Why it matters. The 12Z sounding shows the night's inversion at its strongest; the 00Z one shows the afternoon's mixed layer. Neither shows what happens in between.See Radiosonde, Z, UTC, Run, initGeneral usage
AMDARAircraft observations: Temperature and wind, sometimes humidity, measured automatically by airliners and sent down during climb, cruise and descent.Why it matters. Airliner profiles near busy airports fill the gaps between balloon times and are fed into the models.See Radiosonde, PIREPGeneral usage
CLDNCanadian Lightning Detection Network: A network of ground sensors that locates lightning across Canada by timing and direction-finding the radio pulses it gives off.Why it matters. Lightning is the surest sign a cloud is a thunderstorm. Our thunderstorm probability is checked against lightning data.See Flash, stroke, TS, VCTS, P(TS)General usage
Flash, stroke: A lightning flash is one complete discharge; it can contain several return strokes along the same channel. Flashes can be cloud-to-ground or stay inside or between clouds.Why it matters. In-cloud lightning often increases before cloud-to-ground lightning and before a storm peaks.See CLDN, TS, VCTSGeneral usage
Weather camera: Fixed cameras at airports and passes, including NAV CANADA's network, that show the sky and terrain in several directions, refreshed every few minutes.Why it matters. A camera shows what a sensor can't: cloud on the hills, the edge of a fog bank, or whether a pass is open.See Human observer, Mountain obscurationGeneral usage
SYNOP: A coded surface observation used for weather maps, exchanged worldwide at main synoptic hours (00, 06, 12 and 18Z). It carries more climate detail than a METAR.See METAR, MSLPGeneral usage
Buoys and ship reports: Moored and drifting buoys and ships report wind, pressure, air and sea temperature from over water, where there are no airports.Why it matters. Off the BC coast they are the first to see a Pacific storm's wind and pressure fall.See SYNOP, Advection fogGeneral usage
Road weather stations: Stations run by highway agencies, often with pavement temperature and cameras, that fill gaps between airports.Why it matters. Useful for spotting fog, freezing rain and low cloud in valleys and passes away from airports; their siting isn't to aviation standards.See Weather camera, RepresentativenessGeneral usage
Representativeness: How well one observation stands for the area around it. A valley station, a lakeside station and one on a ridge 20 km apart can all be right and all disagree.Why it matters. A model grid box or a forecast map gives an area average, so it can't match every station. This is part of why verification is hard.See Grid spacing, VerificationGeneral usage
Icing sensor: A small vibrating probe at some automatic stations whose frequency changes as ice builds on it. It helps an automatic station report freezing precipitation.See Present weather sensor, FZRA, FZDZGeneral usage

Cloud and sky

Okta: One eighth of the sky. Cloud amount is counted in oktas.MANAIR p. 65
SKCSky clear: No cloud.MANAIR p. 65
FEW: 1 to 2 oktas of cloud.MANAIR p. 65
SCTScattered: 3 to 4 oktas of cloud.MANAIR p. 65
BKNBroken: 5 to 7 oktas of cloud. A broken layer can be a ceiling.MANAIR p. 65
OVCOvercast: 8 oktas: the whole sky is covered. An overcast layer can be a ceiling.MANAIR p. 65
Ceiling: The height above the ground of the lowest cloud layer where the amounts add up to more than half the sky, or the vertical visibility when something like fog hides the whole sky.MANAIR ch. 8, p. 136
Ceiling proxy: The lowest cloud base in our guidance. It can be low even with little cloud forecast, so read it with the sky amount.This site
VVVertical visibility: How far up you can see into a layer that hides the sky, such as fog or heavy snow. It's used in place of a cloud base.MANAIR ch. 2, p. 68
CB, TCU, ACC, CU: Convective cloud: cumulonimbus, towering cumulus, altocumulus castellanus and cumulus. On a GFA, CB, TCU and ACC imply significant turbulence and icing, and CB also implies low-level wind shear.MANAIR p. 61, 66
LYRLayered: On a GFA, cloud layers less than 2,000 ft apart, described together.MANAIR p. 65
ST, SCStratus, stratocumulus: Low, layered cloud. Stratus is a flat grey sheet that can sit near the ground and give drizzle; stratocumulus is lumpier with gaps.General usage
AC, ASAltocumulus, altostratus: Mid-level cloud, roughly 6,500 to 20,000 ft. Altostratus is a grey sheet that often comes before a warm front; altocumulus is patchy rolls or puffs.General usage
CI, CS, CCCirrus family: High ice cloud, usually above about 20,000 ft. Thin and wispy, rarely a hazard by itself but it can hide lower cloud from a satellite.General usage
NSNimbostratus: A thick, dark layer giving steady rain or snow over a wide area.General usage
Cloud base: The height of the bottom of a cloud layer. In a METAR it is above the ground; on a GFA it is above sea level.General usage
Cloud top: The height of the top of a cloud. Satellites see tops, not bases, so a satellite cloud height is a top height.General usage
Deck: A continuous or near-continuous layer of cloud.General usage
Scud, fractus: Ragged low cloud fragments under a main layer, often in rain. They can lower the effective ceiling quickly.General usage
Obscured sky: Fog, snow or another obscuration hides the sky. A METAR then gives vertical visibility (VV) instead of a cloud base.General usage
The ten cloud types: Clouds are sorted into ten main types by height and shape: high (cirrus, cirrocumulus, cirrostratus), middle (altocumulus, altostratus, nimbostratus), low (stratus, stratocumulus) and the ones that grow upward through several levels (cumulus, cumulonimbus).Why it matters. The type tells you the process: layered cloud means gentle, widespread lift; heaped cloud means rising bubbles and bumpy air.See ST, SC, AC, AS, CI, CS, CC, NS, CB, TCU, ACC, CUGeneral usage
Low, middle and high cloud: Rough height bands for cloud bases. In mid-latitudes, low cloud has its base below about 6,500 ft (2 km), middle cloud from there to roughly 20,000 to 23,000 ft (6 to 7 km), and high cloud above. The bands are lower in the Arctic and in winter.Why it matters. Our ModelMaps low, mid and high cloud maps use model levels like these, so a high-cloud area can sit over clear low levels or over fog.See The ten cloud types, Cloud base · On the site: ModelMaps: Low, Mid and High cloudGeneral usage
Opacity: How much of the sky a cloud layer hides, in oktas. A thin layer can cover the sky while hiding only part of it. Canadian METAR remarks give each layer's type and opacity, such as SC5AC3.Why it matters. Opacity explains why a high thin layer may not count much toward a ceiling while a thick one does.See RMK, Okta, Summation of cloud layers · Learn: Reading a METAR and a TAFGeneral usage
Summation of cloud layers: The rule that each reported layer's amount includes all the cloud below it. SCT010 BKN030 means the low layer alone is scattered and, with the cloud at 3,000 ft added, the total is broken.Why it matters. This is why a layer reported as BKN may be quite thin by itself: it's the total that matters for the ceiling.See Ceiling, FEW, BKN, Opacity · Learn: Reading a METAR and a TAFGeneral usage
Heights on a GFA: Cloud bases and tops on a GFA are in feet above sea level unless marked AGL. METAR and TAF cloud heights are above the ground.Why it matters. Over the Prairies and the foothills the ground itself can be 2,000 to 4,000 ft ASL, so a GFA base of 4,000 ft ASL may be only a few hundred feet above some airports.See ASL, AGL, GFAGeneral usage
Virga: Streaks of rain or snow falling from a cloud that evaporate before reaching the ground.Why it matters. Evaporation chills the air, which can then sink fast: virga under a high-based shower is a classic sign of possible dry microbursts and gusty outflow.See Microburst, DCAPE, Outflow, gust frontGeneral usage
Mammatus: Pouch-like bulges hanging from the underside of a cloud, most often under a thunderstorm anvil.Why it matters. They mark sinking, turbulent air under the anvil; the parent storm is usually strong.See Anvil, CB, TCU, ACC, CUGeneral usage
Shelf cloud, arcus: A low, wedge-shaped cloud along the leading edge of a thunderstorm's cold outflow, where warm air is pushed up over it.Why it matters. It arrives with the gust front: a sudden wind shift, gusts, turbulence and a temperature drop, often minutes before the rain.See Outflow, gust front, SquallGeneral usage
Wall cloud: A lowered, often rotating section of cloud base under the updraft of a thunderstorm, usually toward the rear of the storm.Why it matters. A rotating wall cloud is where tornadoes most often form.See Supercell, FC, +FCGeneral usage
Cap cloud: A cloud that sits over a mountain top like a hat, formed as moist air is lifted over the peak and dissolved as it sinks on the far side.Why it matters. A cap cloud means strong flow over the ridge. Expect downdrafts and turbulence on the lee side and obscured summits.See ACSL, Mountain wave, Orographic cloud · Learn: Mountain waves, rotors and downslope windsGeneral usage
Orographic cloud: Cloud formed by air being forced up over hills or mountains. It stays tied to the terrain even when the wind is strong.Why it matters. Upslope cloud can bury passes and mountain airports while the lee side is clear.See Upslope fog, Cap cloud, Mountain obscurationGeneral usage
Pyrocumulus, pyroCb: Cumulus or cumulonimbus built by the heat of a large wildfire. A pyroCb can make lightning and lift smoke into the stratosphere.Why it matters. Western Canadian fire seasons produce them. Expect smoke, severe turbulence, erratic wind and new fires from lightning nearby.See FG, BR, HZ, FU, CB, TCU, ACC, CU, Wildfire smokeGeneral usage
Contrail: A line of ice cloud behind an aircraft, from water vapour in the exhaust freezing in cold, moist air at high altitude.Why it matters. Long-lasting, spreading contrails mean the upper air is moist enough for cirrus to form.See CI, CS, CCGeneral usage
Cloud streets: Rows of cumulus lined up with the wind, often seen when cold air moves over warmer ground or water.Why it matters. Between the streets the air sinks and is smoother; under them it rises. Lake-effect snow often starts as cloud streets.See Thermals, Lake-effect snowGeneral usage
Open and closed cells: Patterns of low cloud seen on satellite over oceans: closed cells look like a honeycomb of cloud with clear edges; open cells are rings of cloud around clear centres, typical of cold air over warm water.Why it matters. Open cells behind a Pacific front mean showery, unstable air reaching the BC coast; closed cells mean a more solid stratocumulus deck.See Cloud streets, ST, SCGeneral usage
Ragged base: A cloud base that isn't flat but uneven, with shreds hanging below, often in precipitation.Why it matters. The ceiling can change by hundreds of feet within minutes under a ragged base; scud may drift through approach paths.See Scud, fractus, CeilingGeneral usage
Multi-layer cloud: Two or more distinct cloud layers with clear air between them.Why it matters. Satellite sees only the top layer, so fog or low stratus under a high deck can be hidden. Clear gaps between layers may be good places to escape icing.See Cloud top, L2 productGeneral usage
Wildfire smoke: Smoke from forest fires, reported as FU when it cuts visibility. It can sit near the ground under an inversion or spread aloft for thousands of kilometres.Why it matters. Smoke can drop visibility to IFR, especially in valleys in the morning, and cuts daytime heating, which can make forecasts too warm.See FG, BR, HZ, FU, Inversion, Pyrocumulus, pyroCbGeneral usage

Flight categories

VFRVisual flight rules: Our best category: ceiling above 3,000 ft and visibility above 5 SM.This site's bands
MVFRMarginal VFR: Ceiling 1,000 to 3,000 ft, or visibility 3 to 5 SM.This site's bands
IFRInstrument flight rules: Ceiling 500 ft to below 1,000 ft, or visibility 1 SM to below 3 SM.This site's bands
LIFRLow IFR: Ceiling below 500 ft, or visibility below 1 SM. On all four bands, the worse of ceiling and visibility sets the category. These bands are a quick colour guide on this site, not the legal flight rules.This site's bands
VMC, IMC: Visual and instrument meteorological conditions. VMC means the weather is at or above the visibility and cloud-clearance values set for visual flight; IMC means it isn't.Why it matters. Whether a flight can legally continue under visual rules depends on VMC, not on our colour categories.See VFR, VFR weather minima · Learn: Ceilings, visibility and flight categoriesGeneral usage
VFR weather minima: The minimum visibility and distance from cloud required for VFR flight. In Canada they are set in the Canadian Aviation Regulations and depend on the class of airspace, altitude and whether it's day or night.Why it matters. These legal values are not the same as the VFR / MVFR / IFR / LIFR colour bands used on weather sites, including ours.See VMC, IMC, VFR, Special VFR · Learn: Ceilings, visibility and flight categoriesGeneral usage
Special VFR: Permission from air traffic control to fly VFR in a control zone when the weather is below normal VFR minima but above lower special-VFR values set in the regulations.See VFR weather minima, VMC, IMCGeneral usage
VFR OTTVFR over the top: VFR flight above a cloud layer, with rules in Canada about the cloud amount below and the weather forecast at destination.Why it matters. The layer below can thicken or lower while you're above it, so the destination forecast is the key part of the decision.See VFR weather minima, OVCGeneral usage
Approach minima: The lowest height (decision altitude or minimum descent altitude) and visibility or RVR published for an instrument approach. Below them, the approach can't be completed without the required visual references.Why it matters. Ceiling and visibility forecasts matter most near these values; a 200 ft difference in a forecast ceiling can mean landing or diverting.See Ceiling, RVR, Alternate minima · Learn: Ceilings, visibility and flight categoriesGeneral usage
Alternate minima: Weather limits a forecast must meet for an airport to be filed as an alternate under instrument flight rules. In Canada they are published in the Canada Air Pilot and explained in the AIM.Why it matters. This is why TEMPO and PROB groups matter in planning: how they apply to alternates is set out in the AIM.See TEMPO, PROB, Approach minimaGeneral usage
Approach ban: Canadian rules that stop an instrument approach from continuing past a certain point when the reported RVR or visibility is below set values.See RVR, Approach minimaGeneral usage
Marginal: Weather near a limit: not clearly good and not clearly bad. MVFR is the marginal band in flight category colours.Why it matters. Marginal weather is where forecasts bust most often and where small errors change decisions, so it deserves the most checking.See MVFR, VFR, IFRGeneral usage
Category colours: On this site green is VFR, blue MVFR, orange-red IFR and purple LIFR, with grey for unknown. The worse of ceiling and visibility sets the colour.Why it matters. A quick picture of where the weather is poor. It's a guide to our bands, not to the legal minima.See VFR, MVFR, IFR, LIFR · On the site: Airport page, Dashboard · Learn: Ceilings, visibility and flight categoriesThis site
Below alternate: A ceiling or visibility below the aerodrome's alternate limit, the pair of values that a forecast must stay at or above for the airport to be filed as an IFR alternate. Used in TAF work to name the category just under the alternate limit and at or above the landing limit.Why it matters. It is one of the thresholds that decides whether a TAF needs an amendment, and it is why a PROB group often carries exactly this kind of ceiling or visibility.See Below landing, Alternate minima, Amendment criteria, Category ladder (TAF) · Learn: Writing and amending TAFs: the desk viewGeneral usage · This site
Below landing: A ceiling or visibility below the aerodrome's landing limit, the lowest pair of values used as a TAF threshold for that airport. Conditions below landing mean an approach is unlikely to succeed.Why it matters. The lowest rung of the TAF category ladder. When both the forecast and the observation are below it, small differences no longer force an amendment.See Below alternate, Approach minima, Category ladder (TAF) · Learn: Writing and amending TAFs: the desk viewGeneral usage · This site
Category ladder (TAF): The ordered set of ceiling and visibility thresholds a forecaster watches when deciding whether a TAF still fits: 2,500 ft / 6 SM, 1,000 ft / 3 SM, the alternate limit and the landing limit (plus any other published limit). A value exactly at a threshold belongs to the category above it; the category of a report is the lower of its ceiling and visibility categories.Why it matters. Crossing a rung downward on one element is enough to require an amendment; climbing a rung needs both elements to cross.See Below alternate, Below landing, Amendment criteria · Learn: Writing and amending TAFs: the desk viewGeneral usage · This site

Heights, distances and time

AGLAbove ground level: Height above the ground at that spot. Ceilings are given this way.MANAIR p. 58
ASLAbove sea level: Height above mean sea level. Most GFA heights, including cloud bases and tops, icing, turbulence and the freezing level, are given this way, in hundreds of feet.MANAIR p. 58
FLFlight level: A height based on a standard altimeter setting, in hundreds of feet. FL240 is about 24,000 ft.General usage
SMStatute mile: The unit for visibility in Canadian aviation weather. P6SM means more than 6 miles.MANAIR p. 58, 70
KTKnot: Nautical miles per hour, the unit for wind. G marks a gust, so 20G30 is 20 knots gusting to 30.MANAIR p. 58, 72
Z, UTC: Coordinated Universal Time, used for every time in aviation weather and on this site.MANAIR p. 58
QNH: The international name for the altimeter setting: the pressure that makes an altimeter read the airport's elevation on the ground. Canada gives it in inches of mercury, many countries in hPa.See Altimeter setting, Standard pressure, 29.92, Inches of mercuryGeneral usage
Standard pressure, 29.92: 29.92 inches of mercury (1013.25 hPa), the setting used in the standard pressure region so all aircraft share the same reference. Heights read with it are flight levels.Why it matters. Over a deep low the real height of a flight level is lower than its number suggests, and over a high it's higher.See FL, Standard pressure region, Pressure altitudeGeneral usage
Standard pressure region: In Canada, airspace where pilots set 29.92: at and above 18,000 ft ASL in the south, and at all heights in the large northern region outside the altimeter setting region.See Standard pressure, 29.92, FLGeneral usage
Pressure altitude: The height an altimeter shows when set to 29.92. It's the height in the standard atmosphere at which today's pressure would be found.See Standard pressure, 29.92, Density altitude, ISAGeneral usage
Density altitude: The height in the standard atmosphere where the air would be as thin as it is now. Heat, altitude and to a smaller degree humidity raise it. Some Canadian METARs add a density altitude remark on hot days.Why it matters. High density altitude means longer take-off runs and weaker climb. A hot afternoon at Calgary or Cranbrook can feel like a much higher airport.See Pressure altitude, ISAGeneral usage
ISAInternational Standard Atmosphere: A reference atmosphere: 15 °C and 1013.25 hPa at sea level, cooling about 2 °C per 1,000 ft up to the tropopause near 36,000 ft. Instruments and performance charts are built on it.Why it matters. Real air is rarely standard, and the differences are what make altimeters and performance numbers wrong.See Pressure altitude, Density altitude, Lapse rateGeneral usage
Cold temperature error: In air colder than standard, an altimeter reads higher than the aircraft's true height. The colder the air and the higher above the station, the bigger the error. Canada publishes corrections for it.Why it matters. In a Prairie or northern winter the error can be hundreds of feet near minimum altitudes, which matters over terrain and on approaches.See ISA, Standard pressure, 29.92General usage
Inches of mercury: The unit of the altimeter setting in Canada and the United States. 29.92 inHg equals 1013.25 hPa, and 0.01 inHg is about 0.34 hPa.See Altimeter setting, hPaGeneral usage
NMNautical mile: 1,852 m, used for distances in navigation and on GFA charts and SIGMETs. A knot is one nautical mile per hour. A statute mile, used for visibility, is about 1,609 m.See SM, KTGeneral usage
Local time and UTC: Pacific Standard Time is UTC−8, Mountain Standard Time UTC−7 and Central Standard Time UTC−6, each an hour less behind in summer daylight time. Most of Saskatchewan stays on CST all year.Why it matters. Fog and convection follow the sun, not UTC. A 12Z report in Vancouver in winter is 4 a.m. local, well before dawn.See Z, UTC, 00Z and 12ZGeneral usage
Civil twilight: The period just before sunrise and after sunset when the sun is less than 6 degrees below the horizon and there's still useful light. Canadian rules on night flight refer to it.Why it matters. Radiation fog is often thickest around sunrise, just as the morning twilight begins.See Radiation fog, Local time and UTCGeneral usage
Aerodrome elevation: The height of the highest point of the landing area above sea level. It's what the altimeter should read on the ground with the local setting.See ASL, AGL, QNHGeneral usage
Polar night: The weeks around midwinter when the sun doesn't rise above the horizon at high latitudes, north of the Arctic Circle.Why it matters. With no sun, Arctic inversions, ice fog and stratus can last for weeks, and visible satellite images are unavailable.See Ice fog, Inversion, VisibleGeneral usage
ETAEstimated time of arrival: The time a flight is expected to reach a point or its destination, in UTC on aviation products. The route briefing works out an ETA for each stop from your departure time and flight time, and shows the guidance around it.See Z, UTC, Valid time · On the site: Route briefingGeneral usage
ISA deviationISA +/- °C: The actual temperature at a level minus the temperature the ICAO standard atmosphere gives for that pressure altitude (15 °C at sea level, falling 1.98 °C per 1000 ft to -56.5 °C at 36,089 ft and above).Why it matters. Aircraft performance charts are drawn for ISA. Warmer than ISA (ISA +10, say) means higher density altitude, less thrust and lift, longer take-off runs and a lower ceiling; for jets it also changes the true airspeed for a given Mach number.See ISA, Density altitude, Pressure altitude · On the site: Flight-level windsICAO Doc 7488 (standard atmosphere)
True airspeedTAS: The speed of the aircraft through the air around it. It is higher than the indicated airspeed at altitude because the air is thinner (about 2 % more per 1000 ft).Why it matters. Ground speed is true airspeed plus the tailwind (or minus the headwind), so TAS is what the route tool needs to turn winds aloft into a time en route.See Ground speed, Headwind and tailwind component, Upper winds (FD)General usage
Ground speedGS: The speed of the aircraft over the ground: true airspeed corrected for the wind. A crosswind also lowers it slightly, because part of the true airspeed goes into holding the track.Why it matters. Time en route and fuel are set by ground speed, which is why choosing a level with less headwind can save more time than flying faster.See True airspeed, Headwind and tailwind component, CrosswindGeneral usage
Great circle: The shortest path between two points on the Earth. On most map projections it looks curved; its course changes along the way except on north-south routes or along the equator.Why it matters. The route tool follows the great circle between the two airports, so the course (and the head- or tailwind) is worked out piece by piece, not from one starting course.See True airspeedGeneral usage

Fog and visibility

Visibility: How far you can see and identify objects horizontally. In Canada it is reported in statute miles; the prevailing visibility is what applies to at least half of the horizon.General usage
Radiation fog: Fog that forms on clear, calm nights when the ground cools by giving off heat and chills the air just above it to its dew point. It usually lifts or burns off in the morning.General usage
Advection fog: Fog that forms when moist air moves over a colder surface, such as warm air over snow, cold water or a cold coast. It can form at any time of day and persist with wind.General usage
Upslope fog: Fog that forms when moist air is pushed up sloping ground and cools as it rises. Common on the Prairies with easterly winds toward the foothills.General usage
Precipitation fog: Fog that forms when rain falling into cooler air evaporates and saturates it, often ahead of a warm front.General usage
Steam fog, sea smoke: Fog that forms when very cold air moves over much warmer open water, which steams into it.General usage
Ice fog: Fog made of tiny ice crystals, usually in very cold air below about −30 °C, often near towns and airports where engines and chimneys add moisture.General usage
FZFGFreezing fog: Fog of supercooled water droplets below 0 °C. It leaves rime ice on surfaces and aircraft.General usage
MIFG, BCFG, PRFG: Shallow fog, fog patches and partial fog. Shallow fog can leave visibility good at eye height while the ground is hidden.General usage
Dew pointTd: The temperature the air must cool to, at constant pressure, before it is saturated and dew, fog or cloud can form.General usage
SpreadDew-point depression: Temperature minus dew point. A spread of 0 to 1 °C means the air is saturated or nearly so; fog becomes possible when it is small and the air is still cooling.General usage
RHRelative humidity: How close the air is to saturation, as a percentage. 100 % means saturated.General usage
Radiative cooling: The ground losing heat to space at night. It is strongest under clear sky with light wind and dry air above, and weakest under cloud.General usage
Mixing: Wind or daytime heating stirring the low levels. Mixing brings drier, warmer air down and stops shallow fog from forming or breaks it up.General usage
Inversion: A layer where temperature rises with height instead of falling. It traps moisture, fog and smoke beneath it.General usage
Burn-off, lift: Fog ending in the morning as the sun heats the ground and mixes the layer. Fog often lifts into low stratus first before it clears.General usage
Low stratus: A cloud layer with a base a few hundred feet up. From a satellite it looks the same as fog because only the top is seen.General usage
Fog regime: Our label for the kind of setting that could give fog at a place: radiative, advective, low stratus, precipitation-associated or mixed. It describes the environment, not observed fog.This site
Support index: A 0 to 100 score on our fog maps for how many ingredients line up. It is not a probability: 80 does not mean an 80 % chance.This site
Air-column model: Our physical simulation of the lowest 1.5 km over an airport, run hour by hour. It knows about cooling, dew and mixing but cannot see cloud arriving from elsewhere, so it tends to over-forecast fog.This site
Valley fog: Radiation fog that collects in valleys and basins as cold, dense air drains downhill overnight and pools on the valley floor.Why it matters. Interior BC valley airports can sit in fog for days in late autumn and winter while nearby ridges are in sunshine above an inversion.See Radiation fog, Cold pool, Katabatic, drainage wind · Learn: Fog: types and life cycleGeneral usage
Sea fog, marine layer: Fog or low stratus that forms when moist air moves over colder water, then drifts ashore with the sea breeze or onshore flow. The cool, moist layer under a warmer inversion is called the marine layer.Why it matters. On the BC coast it can roll into airports in the evening and early morning, especially in late summer and autumn, and retreat offshore by afternoon.See Advection fog, Sea breeze, land breeze, Low stratus · Learn: Fog: types and life cycleGeneral usage
Saturation: Air that holds as much water vapour as it can at its temperature. Cooling air to its dew point, or adding moisture, saturates it, and more cooling makes droplets: cloud or fog.Why it matters. Every fog forecast is a question of how the air will reach saturation tonight: by cooling, by moistening, or by mixing.See Dew point, Spread, RH · Learn: Fog: types and life cycleGeneral usage
Condensation nuclei: Tiny particles of salt, dust, smoke or pollution that water vapour condenses onto. Without them, droplets would hardly form at all.Why it matters. Polluted or smoky air makes many small droplets, so fog there is often denser for the same amount of water.See Saturation, Wildfire smokeGeneral usage
Dew, frost, hoar frost: Water or ice deposited straight onto cold surfaces when they cool below the dew point (or frost point) of the air. Hoar frost is the feathery ice crystal form.Why it matters. Heavy dew or frost takes water out of the lowest air, which can delay or stop fog. Frost on wings must be removed before take-off.See Radiative cooling, Dew point, Frost on wingsGeneral usage
Decoupling: When the air near the ground goes calm under a strengthening night inversion and stops mixing with the faster wind just above it.Why it matters. Once the surface decouples it can cool quickly and fog can form, even though the wind a few hundred feet up is still strong. That same setup brings low-level wind shear.See Inversion, Radiation fog, LLWS, Nocturnal boundary layer · Learn: Fog: types and life cycleGeneral usage
Nocturnal boundary layer: The shallow layer of air near the ground that cools overnight under clear skies, usually a few hundred feet deep, capped by a radiation inversion.Why it matters. Fog, frost, smoke trapping and the nocturnal low-level jet all live in or just above this layer.See Inversion, Decoupling, Nocturnal low-level jet, PBLGeneral usage
Fog depth: How thick a fog layer is, from the ground to its top. Shallow fog may be tens of feet deep; deep fog can be over 1,000 ft.Why it matters. Deeper fog takes longer for the sun to burn off, and a pilot on the ground may see blue sky through shallow fog while the runway is still below limits.See Burn-off, lift, MIFG, BCFG, PRFG, Fog top · Learn: Fog: types and life cycleGeneral usage
Fog top: The upper surface of a fog layer, often flat and sharp because it lies under an inversion. From above it looks like a white sea filling the valleys.Why it matters. Satellite sees the fog top, not the fog base. Its brightness and temperature help estimate depth.See Fog depth, Night fog difference, InversionGeneral usage
Fog bank, patchy fog: Fog with an edge: a bank that sits offshore or over low ground, or patches that come and go with small changes in wind and ground.Why it matters. Visibility can drop from 15 SM to 1/4 SM in minutes when a bank drifts over the field.See MIFG, BCFG, PRFG, Sea fog, marine layerGeneral usage
Crossover temperature: An old forecasting rule of thumb for radiation fog: if the night's minimum temperature is expected to fall below the lowest afternoon dew point, fog becomes more likely.Why it matters. Simple and sometimes useful, but it ignores wind, cloud and moisture changes overnight. Modern guidance uses many ingredients together.See Radiation fog, Dew point, SpreadGeneral usage
Slant range visibility: How far a pilot can see along the sloping approach path, which can be quite different from the horizontal visibility measured on the ground.Why it matters. In shallow fog the runway can be visible from above but lost in the flare; under a low cloud base, the reverse.See Visibility, RVR, Fog depthGeneral usage
Flight visibility: The visibility forward from the cockpit in flight, as opposed to the ground visibility reported in a METAR.See Visibility, Slant range visibilityGeneral usage
Obscuration: Anything in the air, other than precipitation, that cuts visibility: fog, mist, haze, smoke, dust or blowing snow.See FG, BR, HZ, FU, Obscured sky, BLSN, DRSNGeneral usage
Fog advisory: A public alert from ECCC when dense fog is widespread enough to affect travel. It's not an aviation product.See ECCC public alertsGeneral usage
Wet ground after rain: Moist soil and puddles after daytime rain add water to the lowest air overnight, which raises the dew point near the ground.Why it matters. A clear, calm night after a rainy afternoon is a classic radiation fog night.See Radiation fog, Dew point · Learn: Fog: types and life cycleGeneral usage
Fog life cycle: Fog forms (cooling and moistening reach saturation), matures (deepens and thickens, often near dawn) and dissipates (sun, wind or drier air mixes it out, often lifting first into low stratus).Why it matters. Knowing which stage the fog is in tells you whether the next hour will be worse or better.See Radiation fog, Burn-off, lift, Low stratus · On the site: ModelMaps: Fog formation, persistence and clearing support · Learn: Fog: types and life cycleGeneral usage
Haze layer: A layer of haze or smoke with a sharp top, usually under an inversion. From above, the top looks like a false horizon.Why it matters. Slant visibility through a haze layer, especially into the sun, can be much worse than the reported ground visibility.See Wildfire smoke, Inversion, Slant range visibilityGeneral usage
Visibility in snow: Snow cuts visibility far more than rain at the same rate, because flakes are large and many. Wet, large flakes cut it less per amount of water than fine, dry snow.Why it matters. In snow, visibility often controls the flight category before the ceiling does.See Intensity: −, no sign, +, BLSN, DRSN, Snow showersGeneral usage
Inversion strength: How much warmer the air at the top of a surface inversion is than the air at the ground, often read as the 925 or 850 hPa temperature minus the 2 m temperature. A strong, shallow inversion means the surface has decoupled from the wind aloft.Why it matters. Strong inversions favour radiation fog, ice fog and very low stratus, and they trap the pollution and moisture that feed them.See Inversion, Decoupling, Radiation fog · Learn: Forecasting fog and stratus: radiation, advection and upslopeGeneral usage
Fog-top cooling: Once fog is a few tens of metres deep, its top radiates to space in place of the ground. The layer then cools from the top, mixes gently and deepens, which is why fog thickens through the small hours even as surface cooling slows.Why it matters. Deeper fog takes longer to clear in the morning, and it can lift into a stratus deck instead of burning off.See Radiation fog, Fog depth, Burn-off, lift · Learn: Forecasting fog and stratus: radiation, advection and upslopeGeneral usage
Fog to stratus transition: The usual way radiation fog ends after sunrise: heating from below lifts the fog base so it becomes a low stratus ceiling of a few hundred feet while the visibility underneath improves, then the deck thins and breaks.Why it matters. The airport often goes from LIFR in fog to IFR under stratus for an hour or two before VFR; the TAF should show both steps.See Burn-off, lift, Low stratus, Fog-top cooling · Learn: Forecasting fog and stratus: radiation, advection and upslopeGeneral usage
Arctic stratus: Low, persistent stratus over northern Canada in autumn and early winter, fed by moisture from still-open water and held under a strong Arctic inversion. It can last for days with ceilings of a few hundred feet and little diurnal change.Why it matters. It behaves like advection fog, not radiation fog: sunshine barely helps, and it ends only when the air mass changes or the water freezes over.See Freeze-up season, Advection fog, Inversion, Polar night · Learn: Forecasting fog and stratus: radiation, advection and upslopeGeneral usage
MistBR: Visibility reduced by tiny water droplets, but less than in fog. In Canadian METARs BR is reported when the visibility is 5/8 SM or more (up to 6 SM); below 5/8 SM the same droplets are reported as fog (FG).Why it matters. Mist on a calm, clear night often thickens into fog as the air keeps cooling, so a METAR with BR and a small temperature–dew point spread is an early warning.See FG, BR, HZ, FU, FZFG, Visibility, SpreadGeneral usage (Canadian METAR practice)

Weather and hazards

RA, SN, DZ, PL: Rain, snow, drizzle and ice pellets. - before a code means light and + means heavy. SH means showers.General usage
FZRA, FZDZ: Freezing rain and freezing drizzle, which freeze on contact and can ice an aircraft quickly.General usage
FG, BR, HZ, FU: Fog, mist, haze and smoke: things that reduce visibility without being precipitation. Fog is denser than mist.General usage
BLSN, DRSN: Blowing snow and drifting snow, which wind lifts from the ground.General usage
TS, VCTS: A thunderstorm at the airport, and one in the vicinity.General usage
Low-level jetL LVL JET: A band of strong wind not far above the ground. A GFA always shows one at 50 KT or more, and may show one at 35 to 45 KT if the forecaster expects significant turbulence or shear. Speed is rounded to 5 KT. MANAIR sets no height range for it.MANAIR p. 80
LLWSLow-level wind shear: A sharp change in wind within 1,500 ft of the ground, not caused by convection. Forecaster guidelines are a vector change over 25 KT within 500 ft, 40 KT within 1,000 ft, or 50 KT within 1,500 ft.MANAIR p. 22 to 23
Icing: RIME, MX, CLR: The type of ice forming on an aircraft: rime, mixed or clear. Intensity is judged by its effect on the aircraft. Light can become a problem over a long flight, moderate is potentially hazardous even briefly, and severe needs immediate diversion.MANAIR p. 76 to 77
Turbulence: CAT, MECH, MTW: Clear-air, mechanical (wind over rough ground) and mountain wave turbulence. Intensity is judged by its effect: light, moderate or severe. There are no numeric limits in MANAIR.MANAIR p. 78 to 79
FZLVLFreezing level: The height where the temperature falls to 0 °C. An above-freezing layer (AFL) is warmer air above it, which can lead to freezing rain.MANAIR p. 80
ISOL, OCNL, FRQ: How much of an area has convective cloud: up to 25 %, over 25 and up to 50 %, and over 50 %.MANAIR p. 67
LCA, PTCHY, XTNSV: How much of an area has other weather: local (up to 25 %), patchy (over 25 and up to 50 %) and extensive (over 50 %).MANAIR p. 68
ECCC public alerts: Environment and Climate Change Canada's alerts for the public: special weather statements, advisories, watches (conditions are favourable) and warnings (the event is happening or about to).Why it matters. Public warnings are for people on the ground, but a blizzard, freezing rain or severe thunderstorm warning near your airport is a clear sign of trouble for flying too.See SIGMET, AIRMETGeneral usage
Wind shift: A sudden change in wind direction, often with a change in speed: behind a cold front, at a sea breeze or outflow boundary, or when a valley wind reverses.Why it matters. A wind shift can turn a headwind into a tailwind on final, or switch the runway in use.See Front, Outflow, gust front, Sea breeze, land breezeGeneral usage
FROPAFrontal passage: The moment a front passes a point. A cold frontal passage typically brings a wind shift, a pressure rise, a temperature drop and often showers or a squall line.See Front, Wind shiftGeneral usage
Tornado: A violently rotating column of air reaching from a thunderstorm to the ground. Canada averages dozens a year, most on the Prairies, in Ontario and in Quebec, mostly in summer.See FC, +FC, Supercell, STPGeneral usage
Waterspout: A tornado-like vortex over water. Weaker fair-weather waterspouts form under growing cumulus over warm lakes, often in late summer and autumn.See FC, +FC, TornadoGeneral usage
Blizzard: Strong wind with snow or blowing snow that keeps visibility very low for hours. ECCC sets warning criteria by region, and they are lower in the Arctic than in the south.Why it matters. Prairie and Arctic blizzards can close airports for a day or more, often under a sky that's only partly cloudy.See BLSN, DRSN, Whiteout, ECCC public alertsGeneral usage
Snow squall: A short, intense burst of heavy snow and gusty wind, often with a sharp drop in visibility to near zero, along an arctic front or in lake-effect bands.Why it matters. A snow squall can take visibility from VFR to near zero in a few minutes and pass just as fast.See Lake-effect snow, Arctic front, Snow showersGeneral usage
LTG, LTGIC, LTGCG: Remark abbreviations for lightning, in cloud (IC), cloud to ground (CG), cloud to cloud (CC) or cloud to air (CA), often with a direction such as LTG DSNT W.See Flash, stroke, TS, VCTSGeneral usage
Lightning strike to aircraft: Aircraft are struck quite often, mostly near the freezing level in or near cloud with precipitation, and not always in a thunderstorm the crew could see.Why it matters. Strikes rarely bring an aircraft down but can damage avionics, composite structure and radomes, and they can startle crews.See Flash, stroke, FZLVLGeneral usage
Wake turbulence: The pair of spinning vortices trailing from an aircraft's wingtips. It isn't weather, but weather decides how long it lasts and where it drifts.Why it matters. In light wind the vortices can sit over the runway or drift onto a parallel one; a light crosswind of a few knots is the worst case.See Crosswind, Turbulence: CAT, MECH, MTWGeneral usage
Headwind and tailwind component: The part of the wind blowing straight along the runway or track. Against the aircraft it's a headwind, from behind a tailwind. The rest is the crosswind component.See Crosswind, GustGeneral usage
Freezing spray: Sea spray that freezes on ships and structures in strong wind over open water when the air is well below freezing.Why it matters. A marine hazard on the BC coast and in the Arctic. It also signals cold, strong offshore wind.See Arctic outflowGeneral usage
Heavy rain: Rain falling fast, reported as +RA or +SHRA. It cuts visibility, can flood runways and is often tied to convective cells or a strong Pacific atmospheric river on the coast.See RA, SN, DZ, PL, Atmospheric riverGeneral usage
Graupel: Soft, white pellets formed when supercooled droplets freeze onto falling snowflakes. Reported as GS.Why it matters. Graupel means supercooled water and riming in the cloud, so there's icing aloft, and often convection.See GR, GS, Rime iceGeneral usage
Stratiform and convective precipitation: Stratiform precipitation falls steadily from layered cloud over a wide area; convective precipitation comes in showers from cumulus and cumulonimbus, heavier but patchier.Why it matters. Steady precipitation means steady conditions you can plan on; showers mean conditions that swing quickly between good and bad.See NS, Snow showers, CB, TCU, ACC, CUGeneral usage
Evaporative cooling: Cooling of the air when rain, snow or cloud evaporates or sublimates into it, because that takes heat from the air.Why it matters. It powers downbursts under high-based showers and can turn rain to snow when precipitation begins in dry air.See Wet-bulb temperature, Virga, DCAPEGeneral usage
FRAMFreezing Rain Accumulation Model: A method from the US National Weather Service (Sanders and Barjenbruch, 2016) that turns freezing-rain rate, wind and wet-bulb temperature into an estimate of ice accretion on wires and trees (radial ice). Not all freezing rain freezes: some runs off, more so with heavy rates and temperatures near 0 °C.See Ice accretion (radial and flat), Ice storm, FZRA, FZDZ, SPIA index · On the site: Freezing rain / ice stormsGeneral usage (NWS)
Ice accretion (radial and flat): How much ice builds up in freezing rain. Flat ice is the thickness on a horizontal surface; radial ice is the equivalent uniform thickness around a wire or branch, which is what loads power lines. Around 6 mm of radial ice starts to break branches and is a common warning threshold; 12 mm or more can bring down power lines, especially with wind; 25 mm or more is a severe ice storm.See FRAM, Ice storm, Glaze, SPIA indexGeneral usage
SPIA indexSperry–Piltz Ice Accumulation index: A 0 to 5 scale that rates the likely damage from an ice storm, mainly to power lines and trees, from the forecast ice accumulation and the wind during and after the storm.See Ice accretion (radial and flat), FRAM, Ice stormGeneral usage (spia-index.com)

Icing

Supercooled liquid waterSLW: Cloud droplets that stay liquid below 0 °C. They freeze when they hit an aircraft, which is how most airframe icing happens.General usage
SLDSupercooled large drops: Freezing drizzle and freezing rain sized drops. They can run back past the protected leading edges before freezing and are especially dangerous.General usage
Rime ice: Rough, milky ice from small droplets freezing almost instantly. Most common from about −10 to −20 °C, but it can form at any temperature below freezing.General usage
Clear ice: Hard, glassy ice from large drops that spread before freezing. Most common just below 0 °C and in freezing rain or drizzle; harder to see and to shed than rime.General usage
Mixed ice: A mix of rime and clear ice, typical of a range of droplet sizes.General usage
Warm noseAbove-freezing layer: A layer of above-freezing air aloft with colder air below. Snow melts in it and then refreezes lower down, giving freezing rain or ice pellets.General usage
Bourgouin method: A way to sort snow, rain, ice pellets and freezing rain from the size of the warm and cold layers in a model sounding. Used on our freezing rain / ice pellets maps.General usage · This site
Icing potential: Our 0 to 1 index of how favourable the model air is for icing. It is uncalibrated and has no intensity: it does not say light, moderate or severe.This site
FZDZ environment: A shallow, mostly liquid cloud with tops warmer than about −10 to −12 °C and little ice above it: the classic setting for freezing drizzle. An environment, not observed drizzle.General usage · This site
Trace icing: Ice that is just noticeable, building slowly enough that it isn't a hazard unless it goes on for a long time. Above it come light, moderate and severe.See Icing: RIME, MX, CLR, Icing intensityGeneral usage
Icing intensity: How fast ice builds and how much it affects the aircraft: trace, light, moderate or severe. It depends on the water content, drop size, temperature, airspeed and the shape of the aircraft's surfaces, so two aircraft in the same cloud can report different intensities.Why it matters. Forecasts give a potential for the atmosphere; a PIREP tells you what one aircraft actually met.See Icing: RIME, MX, CLR, PIREP, Liquid water content · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Liquid water content: How much liquid water a cloud holds, in grams per cubic metre. Most cloud has well under 1 g/m³; strong convective updrafts can hold several.Why it matters. More supercooled water means faster ice build-up. Cumuliform cloud usually ices faster than thin layered cloud.See Supercooled liquid water, Icing intensity, Drop size, MVD · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Drop size, MVD: How big cloud drops are, often summarised as the median volume diameter in micrometres. Ordinary cloud drops are 10 to 20 µm; drizzle drops are 50 to 500 µm and raindrops larger.Why it matters. Small drops freeze near the leading edge where ice protection works; big drops (SLD) run further back before freezing.See SLD, Supercooled liquid water, Liquid water content · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Ice-crystal processBergeron process: In a cloud with both ice crystals and supercooled droplets, the crystals grow at the droplets' expense because ice holds water vapour more tightly. The crystals become snow and fall out.Why it matters. Clouds with plenty of ice crystals, typically colder than about −15 °C or seeded from above, tend to have less supercooled water and less icing.See Supercooled liquid water, Seeder–feeder, Warm-rain process · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Warm-rain processCollision–coalescence: Cloud droplets bumping into each other and merging until they're big enough to fall as drizzle or rain, without any ice involved. It can work in supercooled cloud too.Why it matters. This is how most freezing drizzle forms: a shallow, ice-free stratus deck with tops warmer than about −10 °C grows drizzle drops by collision, and they fall into sub-zero air.See FZDZ environment, SLD, Ice-crystal process · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Seeder–feeder: Ice crystals or snow falling from a higher cloud (the seeder) into a lower cloud (the feeder), where they grow and often use up its supercooled water.Why it matters. A lower deck under falling snow often has less icing than the same deck with clear air above it.See Ice-crystal process, Multi-layer cloudGeneral usage
Melting layer: A layer warmer than 0 °C aloft where falling snow melts. Below it, a sub-zero layer can refreeze the drops into ice pellets or leave them as supercooled rain that freezes on contact.Why it matters. The depth and warmth of the melting layer and the cold layer under it decide whether you get snow, ice pellets or freezing rain.See Warm nose, Bourgouin method, Refreezing layer · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Refreezing layer: The sub-zero layer near the ground under a melting layer. A deep, cold one refreezes melted drops into ice pellets; a shallow one leaves them liquid, so they freeze on impact as freezing rain.See Melting layer, Warm nose, Ice pellets or freezing rain · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Ice pellets or freezing rain: Both start as snow that melts in a warm layer aloft. If the cold layer below is deep and cold enough, the drops refreeze into pellets that bounce; if not, they land as supercooled rain and glaze everything.Why it matters. A small change in the depth of the cold layer can switch one to the other, which is why forecasts of the exact type are hard and often shown as mixed.See Melting layer, Refreezing layer, FZRA, FZDZ · On the site: ModelMaps: Freezing rain / ice pellets · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Multiple freezing levels: When a warm layer aloft sits above sub-zero air near the ground, the temperature crosses 0 °C more than once on the way up.Why it matters. The single freezing-level number can mislead: there may be a warmer escape layer aloft and freezing rain below it.See FZLVL, Warm nose, Melting layer · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Runback ice: Water that doesn't freeze on the protected leading edge but runs back and freezes behind it, often as a ridge.Why it matters. Ridges behind the protected area disturb the airflow badly and can't be shed. They are typical of large drops and temperatures near 0 °C.See SLD, Clear ice, Ice ridgeGeneral usage
Ice ridge: A raised line of ice along or behind a wing's leading edge.Why it matters. Even a small ridge can cut lift sharply and raise stall speed.See Runback ice, Tailplane stallGeneral usage
Tailplane stall: An ice-contaminated horizontal tail can stall before the wing, typically when flaps are extended, causing a sudden nose-down pitch.See Ice ridge, Icing: RIME, MX, CLRGeneral usage
Carburettor icing: Ice forming inside a carburettor as fuel evaporation and the venturi cool the incoming air. It can happen with outside temperatures well above freezing when the air is humid.Why it matters. It doesn't need cloud or freezing weather: a humid summer morning is a classic setting.See Dew point, RHGeneral usage
Ice crystal icing: Ice building inside jet engines or on probes when flying through very high concentrations of small ice crystals, usually near deep convection at high altitude. Radar often shows little there.See CB, TCU, ACC, CU, AnvilGeneral usage
Known icing approval: Certification that allows an aircraft to fly into forecast or reported icing, because it has approved ice protection. It covers ordinary cloud drops; SLD is often outside it.See SLD, Icing: RIME, MX, CLRGeneral usage
Getting out of icing: The usual ways out are a climb or descent into air warmer than 0 °C or colder than the icing band, leaving cloud, or turning back to where conditions were better.Why it matters. Knowing the freezing level, cloud tops and any warm layer before departure tells you which escape exists.See FZLVL, Cloud top, Multiple freezing levels · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Supercooled tops: Cloud tops colder than 0 °C that are still made of liquid drops, which satellites can detect by their reflectivity and temperature.Why it matters. Liquid tops in the −5 to −15 °C range over a large area are a strong icing signal, especially for freezing drizzle.See Supercooled liquid water, Cloud-top phase · On the site: Satellite: Supercooled liquid tops · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Partial thickness: The depth of a shallow pressure layer, usually 1000–850 hPa and 850–700 hPa, as a measure of its mean temperature. Thick layers are warm. Used to spot a warm layer aloft over a cold surface layer when deciding between snow, ice pellets, freezing rain and rain.Why it matters. Two numbers tell you whether the sounding has a warm nose and whether the surface layer is cold enough to matter.See Thickness, Warm nose, Refreezing layer · Learn: Freezing precipitation: sounding signatures and nowcastingGeneral usage
Positive and negative area: The energy a falling particle gains for melting in a layer warmer than 0 °C (the positive area) and loses for refreezing in a sub-zero layer below it (the negative area), both read from a sounding in J/kg. Comparing the two is the basis of area methods for precipitation type.Why it matters. A warm nose alone does not decide: a large negative area below it turns the melted drops back into ice pellets.See Bourgouin method, Warm nose, Refreezing layer, Ice pellets or freezing rain · Learn: Freezing precipitation: sounding signatures and nowcastingGeneral usage
Melting-induced cooling: Melting snow takes heat from the air it falls through, so heavy precipitation can cool a warm nose toward 0 °C and build an isothermal layer. The warm layer erodes from within and the precipitation type can swing back from freezing rain to ice pellets or snow.Why it matters. It is one reason freezing rain events sometimes end as snow without any cold advection.See Warm nose, Latent heat, Wet-bulb temperature · Learn: Freezing precipitation: sounding signatures and nowcastingGeneral usage
Ice nuclei, glaciation: Particles that let supercooled droplets freeze. They are scarce at temperatures warmer than about −10 °C, so clouds with tops warmer than that often stay all liquid; colder tops glaciate, the ice crystals grow at the droplets' expense and the cloud snows itself out.Why it matters. Cloud-top temperature is the quickest guide to whether a stratus deck will give freezing drizzle or light snow.See Supercooled tops, Ice-crystal process, FZDZ environment · Learn: Freezing precipitation: sounding signatures and nowcastingGeneral usage
Stratocumulus icing: Icing in the shallow stratocumulus and stratus decks that cap arctic and post-frontal air masses: liquid water concentrated near the cloud top, temperatures of −5 to −15 °C, and little ice to remove it. Modest water content but very long exposure for an aircraft held in the deck.See Supercooled liquid water, Supercooled tops, Rime ice, FZDZ environment · Learn: Icing and turbulence in depth: SLD, stratocumulus, waves and CATGeneral usage
Freezing level0 °C level: The height where the air temperature falls to 0 °C, usually given in feet above sea level. When the air near the ground is already below freezing, the freezing level is at the surface. With a warm layer aloft there can be more than one; the flight-level winds page shows the lowest.Why it matters. Above the freezing level, cloud and precipitation can hold supercooled water that freezes on the airframe. Knowing it tells a pilot how high they can climb in cloud before icing becomes possible, and whether a descent will get out of it.See ISA, Lapse rate, Refreezing layer · On the site: Flight-level windsGeneral usage (MANAIR, WMO)

Winter operations

Frost on wings: A thin layer of frost on aircraft surfaces from a clear, cold night, or from cold-soaked fuel cooling the wing below the frost point.Why it matters. Even frost as rough as fine sandpaper can sharply cut lift. Canadian rules require critical surfaces to be clean before take-off.See Dew, frost, hoar frost, Clean aircraft concept, Cold-soaked fuel frostGeneral usage
Clean aircraft concept: The rule that no one may take off with frost, ice or snow adhering to critical surfaces such as wings, tail and control surfaces.See Frost on wings, De-icing and anti-icing, Holdover timeGeneral usage
Cold-soaked fuel frost: Frost or ice on wings over fuel tanks after a flight at cold altitudes, because the fuel stays well below freezing even when the air on the ground is above 0 °C.Why it matters. It can form in humid, above-freezing weather that otherwise looks harmless.See Frost on wingsGeneral usage
De-icing and anti-icing: De-icing removes frost, ice and snow from the aircraft, usually with heated fluid. Anti-icing coats the clean aircraft with thicker fluid that holds off new contamination for a while.See Holdover time, Clean aircraft concept, Type I, II, IV fluidsGeneral usage
Type I, II, IV fluids: De-icing fluids. Type I is thin, heated and used mainly to remove contamination. Types II and IV are thickened and stay on the wing to protect it until take-off, then shear off in the take-off run.See De-icing and anti-icing, Holdover timeGeneral usage
Holdover time: How long an anti-icing fluid is expected to protect the aircraft, taken from tables by fluid type, temperature and the type and rate of precipitation.Why it matters. Holdover times are much shorter in freezing rain, freezing drizzle, ice pellets and heavy snow, so the precipitation type and rate in the METAR matter directly.See De-icing and anti-icing, Type I, II, IV fluids, Intensity: −, no sign, +General usage
Runway condition report: A report of what covers each third of a runway (snow, slush, ice, water, frost and how deep) and how slippery it is, using the international Global Reporting Format that Canada adopted in 2021.See RWYCC, Runway contaminants, CRFIGeneral usage
RWYCCRunway condition code: A number from 6 (dry) down to 0 (very slippery wet ice, for example) given to each third of a runway, based on the contaminant, its depth and the temperature.See Runway condition report, Runway contaminantsGeneral usage
CRFICanadian Runway Friction Index: A Canadian measure of runway friction from a decelerometer in a vehicle, used with tables to estimate landing distance on contaminated runways.See Runway condition report, RWYCCGeneral usage
Runway contaminants: Anything on the runway surface that changes braking and acceleration: dry snow, wet snow, slush, compacted snow, ice, wet ice, frost and standing water.Why it matters. Temperatures near 0 °C are the worst: ice with a film of water on it is far more slippery than ice at −15 °C.See Runway condition report, RWYCCGeneral usage
Whiteout: When snow cover and uniform cloud or blowing snow make light come from every direction, so the horizon, slopes and depth disappear.Why it matters. Pilots can lose all sense of height and attitude over snow even with fair visibility. It's a known killer in the Arctic and in mountain flying.See Flat light, BLSN, DRSN, BlizzardGeneral usage
Flat light: Diffuse light under overcast cloud over snow that removes shadows and contrast, making terrain hard to judge even though visibility is reported as good.See WhiteoutGeneral usage
Blowing snow threshold: The wind speed at which snow on the ground starts to lift. Fresh, dry, cold snow lifts easily; old, crusted or wet snow needs much stronger wind or doesn't lift at all.Why it matters. The same 25 KT wind can give near-zero visibility the day after a snowfall and nothing a week later. That's why blowing snow forecasts look at the snow's age and temperature, not just the wind.See BLSN, DRSN, BlizzardGeneral usage
Lake-effect snow: Bands of heavy snow that form when very cold air crosses a large unfrozen lake, picking up heat and moisture, then dump it on the downwind shore.Why it matters. Bands can be only 10 to 30 km wide: one airport gets near-zero visibility while the next is in sunshine. It happens downwind of the Great Lakes, and of large Prairie and northern lakes until they freeze.See Snow squall, Cloud streetsGeneral usage
Arctic front: The leading edge of very cold, dense arctic air pushing south. It often has a sharp temperature drop and a wind shift, and can bring snow squalls.Why it matters. Behind it, temperatures can fall 10 to 20 °C in hours, with blowing snow on the Prairies and ice fog in the north.See Front, Air mass, Arctic outflowGeneral usage
Arctic outflow: Cold arctic air from the interior pouring through BC's coastal gaps and inlets, such as the Fraser Valley and the fjords, out to the coast.Why it matters. It brings strong, gusty, very cold northeast winds and low wind chills to the coast, with freezing spray at sea and big turbulence near the gaps. When moist Pacific air then overruns the cold air, freezing rain or heavy snow can follow.See Gap wind, Arctic front, Freezing spray · Learn: Prairie inversions, chinooks and upslopeGeneral usage
Wet-bulb temperature: The lowest temperature air can be cooled to by evaporating water into it. It lies between the temperature and the dew point.Why it matters. When rain or snow starts falling into dry air, evaporation cools the air toward the wet-bulb temperature, which can turn rain to snow at the surface within an hour.See Dew point, Rain–snow line, Wet-bulb zeroGeneral usage
Rain–snow line: The boundary on the ground between rain and snow. It depends on the whole temperature profile, not just the surface temperature: snow can reach the ground at +2 °C or more in dry air or heavy precipitation.See Wet-bulb temperature, Thickness, Melting layerGeneral usage
Frost point: The temperature at which air becomes saturated over ice. Below 0 °C it's a little higher than the dew point.Why it matters. Frost and ice fog can form even when the temperature hasn't quite reached the reported dew point.See Dew point, Dew, frost, hoar frost, Ice fogGeneral usage
Extreme cold operations: Operating in temperatures far below standard: batteries, oils and seals suffer, ice fog forms from engine exhaust, and altimeters read too high.See Ice fog, Cold temperature errorGeneral usage
Snow cover: Snow on the ground reflects most sunlight and gives off heat efficiently at night, so the air above it stays colder by day and cools faster at night.Why it matters. Models that get the snow cover wrong get temperatures, inversions and fog wrong too.See Radiative cooling, InversionGeneral usage
Snow showers: Snow from convective cloud, reported as SHSN: bursts of heavier snow with clearer spells between.Why it matters. Visibility in snow showers can swing from good to below a mile within minutes, so a single METAR can mislead.See Snow squall, MI, BC, PR, DR, BL, SH, TS, FZGeneral usage
Glaze: The smooth, clear coat of ice left on the ground, wires and aircraft by freezing rain or freezing drizzle.See FZRA, FZDZ, Clear ice, Ice stormGeneral usage
Ice storm: A long period of freezing rain that builds a thick glaze on everything, bringing down trees and power lines and closing airports.See Glaze, Warm nose, Ice pellets or freezing rain · Learn: Airframe icing, the warm nose and freezing drizzleGeneral usage
Snow-to-liquid ratioSLR: Centimetres of snow per centimetre of melted water. About 10:1 is a convenient average; wet snow near 0 °C runs 5 to 8:1, cold calm snow 15 to 25:1 or more. It depends on the temperature where the crystals grow, on riming and on wind.Why it matters. Visibility, accumulation on runways and the blowing-snow threshold all depend on it.See Visibility in snow, BLSN, DRSN, Snow cover · Learn: Winter operations: blowing snow, snow ratios, ice fog and contaminated runwaysGeneral usage
DRSNDrifting snow: Snow raised by the wind to less than two metres above the ground, so the visibility at eye level is not reduced. Above two metres it becomes blowing snow, BLSN, which does reduce visibility.See BLSN, DRSN, Blowing snow threshold, Whiteout · Learn: Winter operations: blowing snow, snow ratios, ice fog and contaminated runwaysGeneral usage
Dendritic growth zoneDGZ: The layer of a cloud where the temperature is about −12 to −18 °C. Ice crystals grow fastest there and as branching, star-shaped dendrites, which stack loosely and make light, fluffy snow.Why it matters. A deep, saturated DGZ with rising air in it is the classic recipe for heavy snow with high snow-to-liquid ratios (15–25:1); a DGZ that is dry or has no lift gives smaller crystals and denser snow.See Snow-to-liquid ratio, Vertical motion, omega, Saturation, Kuchera snow ratio · On the site: Winter Storm LabGeneral usage
Kuchera snow ratio: A rule of thumb that sets the snow-to-liquid ratio from the warmest temperature in the column between the ground and 500 hPa: colder columns give higher ratios (fluffier snow), columns near 0 °C give lower ratios (wetter snow). It is operational practice, not a published, peer-reviewed method.Why it matters. It captures the big difference between cold dry snow and wet snow with one number, but cannot see where the snow actually grows, so it often runs too high in cold, shallow cloud.See Snow-to-liquid ratio, Physically based snow ratio, Dendritic growth zone · On the site: Winter Storm LabOperational practice
Physically based snow ratioCobb–Waldstreicher style: A snow-to-liquid ratio built from the crystal types that grow at each cloudy, below-freezing level of the model column (highest in the −12 to −18 °C dendritic zone), weighted toward the levels where the most snow is made (strongest upward motion, or deepest cloud), then reduced for melting aloft and wind.Why it matters. It responds to where the snow actually grows, so it does better than single-temperature rules on fluffy-snow days, but it depends on the model getting the cloud and the lift right.See Kuchera snow ratio, Dendritic growth zone, Snow-to-liquid ratio, Vertical motion, omega · On the site: Winter Storm LabCobb and Waldstreicher 2005

Turbulence and wind

Chop: Light, rapid bumps without much change in height or attitude, as opposed to turbulence that causes larger changes.General usage
Mountain wave: Waves in the air downwind of a mountain range when strong wind blows across it in stable air. They can give strong up- and downdrafts, turbulence and lens-shaped clouds.General usage
Rotor: A rolling eddy under a mountain wave, downwind of the ridge and near ridge height or below. Rotors can give severe turbulence.General usage
Lee wave: A mountain wave that repeats downwind of the ridge, often marked by rows of lenticular cloud.General usage
ACSLLenticular cloud: Smooth, lens-shaped cloud that sits still over or downwind of mountains: a sign of mountain waves.General usage
Downslope windstorm: Very strong, gusty wind on the lee side of mountains, such as a strong chinook along the Alberta foothills.General usage
Chinook: A warm, dry, often gusty westerly wind coming down the east side of the Rockies. It can bring wind shear and mechanical turbulence near the foothills.General usage
Jet stream: A narrow band of very strong wind near the top of the troposphere, often 100 KT or more. Clear-air turbulence is common near its edges.General usage
Tropopause: The boundary between the weather layer (troposphere) and the stable stratosphere above, roughly 25,000 to 40,000 ft over Canada. The jet stream sits just below it.General usage
Tropopause fold: Where stratospheric air dips steeply down into the troposphere near a jet, often with turbulence.General usage
Wind shear: A change in wind speed or direction over a distance, up or across. Strong vertical shear causes turbulence; near the ground it affects takeoff and landing.General usage
Richardson numberRi: A ratio comparing how stable the air is with how strongly the wind shears. Low values, below about 0.25, mean turbulence can break out.General usage
Ellrod indexTI1, TI2: A clear-air turbulence index from vertical wind shear and how the wind field stretches and squeezes. Higher values point to more risk.General usage
TKETurbulent kinetic energy: The energy in small swirling motions, which some models calculate directly. Higher TKE means rougher air.General usage
EDREddy dissipation rate: An aircraft-independent measure of turbulence. Not shown on this site yet.General usage
Froude number: Compares wind speed across a ridge with the air's stability and the ridge height. Around 1, mountain waves are strongest; well below 1 the air tends to flow around or be blocked.General usage
Scorer parameter: A measure that falls with height when waves can be trapped near the ground and travel far downwind as lee waves.General usage
CITConvectively induced turbulence: Turbulence in and around thunderstorms, including clear air above and beside them.General usage
PBLPlanetary boundary layer: The lowest part of the atmosphere, mixed by the ground's friction and heating. Deep on sunny afternoons, shallow on calm nights.General usage
Thermalsw*: Rising bubbles of warm air on sunny days that make the ride bumpy below cloud base. w* is the typical speed of those updrafts.General usage
Mechanical turbulence: Bumpy air near the ground when strong wind flows over rough terrain, trees or buildings.General usage
Crosswind: The part of the wind blowing across the runway.General usage
Nocturnal low-level jet: A band of strong wind a few hundred to a few thousand feet up that often develops at night over the Prairies when the ground layer decouples. It can give low-level wind shear at dawn.General usage
Light, moderate, severe, extremeTurbulence intensity: Turbulence is graded by its effect. Light: small, erratic bumps, loose items stay put. Moderate: stronger changes in height or attitude, people strain against belts, walking is hard, but the aircraft stays in control. Severe: large, abrupt changes, momentary loss of control, loose objects thrown about. Extreme: the aircraft is violently tossed and nearly impossible to control.Why it matters. The same air can be moderate for a small aircraft and light for an airliner, so PIREPs always include the aircraft type.See Turbulence: CAT, MECH, MTW, PIREP, EDRGeneral usage
Clear-air turbulenceCAT: Turbulence at high altitude away from visible cloud or storms, usually in strong vertical wind shear near the jet stream or the tropopause.Why it matters. There's nothing to see on radar or out the window, so forecasts of shear and PIREPs are the main warning.See Jet stream, Wind shear, Richardson number, Ellrod index · On the site: ModelMaps: Turbulence potential by flight levelGeneral usage
Kelvin–Helmholtz wavesBillows: Breaking waves that form where one layer of air slides fast over another, like wind over water. Sometimes visible as a row of curled cloud crests.Why it matters. They are one of the main ways shear turns into clear-air turbulence. The Richardson number tells you when they can form.See Richardson number, Clear-air turbulence, Wind shearGeneral usage
Gravity waves: Waves in stable air, where gravity pulls displaced air back and it overshoots, much like ripples on a pond. Mountains, storms and jet streams all set them off.Why it matters. Most mountain-wave and much clear-air turbulence comes from gravity waves growing and breaking.See Mountain wave, Lee wave, Stable, unstable, neutralGeneral usage
Frontal turbulence: Bumps where warm and cold air meet at a front, from the lifting, the wind shift and the shear across the frontal zone, strongest with fast-moving cold fronts.See Front, Wind shear, FROPAGeneral usage
Veering, backing: A wind veers when its direction turns clockwise (south to west, for example) and backs when it turns counter-clockwise. The terms are used both for changes over time and with height.Why it matters. Wind veering with height usually means warm air moving in; backing with height means cold air moving in. Veering with height near the ground also helps storms rotate.See Warm and cold advection, Hodograph, SRHGeneral usage
Geostrophic wind: The wind that would blow if the pressure-gradient force and the Earth's rotation were exactly balanced. It blows along the isobars or height contours, faster where they're closer together.Why it matters. Above the friction layer, real winds are close to it, which is why tight contours on a 500 hPa chart mean strong winds.See Isobar, Friction layer, Height (geopotential)General usage
Friction layer: The lowest part of the atmosphere, roughly the first 1,000 to 3,000 ft, where the ground slows the wind and turns it to cross the isobars toward low pressure.Why it matters. Surface winds are slower and backed compared with the wind a few thousand feet up. On a clear night the difference grows as the surface decouples.See PBL, Geostrophic wind, DecouplingGeneral usage
Daily wind cycle: Over land, surface winds usually pick up during the day as heating mixes faster air down from above, and drop off in the evening as the air near the ground stabilises.Why it matters. Afternoon gusts and evening calm on sunny days are this cycle at work. Gust forecasts lean on how deep the afternoon mixing gets.See Mixing, PBL, GustGeneral usage
Updraft, downdraft: Rising and sinking currents of air: in thermals, in storms, over and behind mountains, and in waves.See Thermals, Microburst, Mountain waveGeneral usage
Gust spread: The difference between the gust and the mean wind, for example 10 KT in 15G25KT.Why it matters. A large spread means turbulent, mixing air near the ground and big airspeed changes on approach. Many pilots add part of it to their approach speed.See Gust, Mean wind, Mechanical turbulenceGeneral usage
Isotach: A line on a map joining places with the same wind speed, often shaded on jet-stream charts.See Jet stream, Jet streak · On the site: ModelMaps: 250 hPa wind (jet stream)General usage
Wind gradient near the ground: The increase of wind speed with height in the first few hundred feet, caused by friction at the surface.Why it matters. On final, the headwind often drops as you descend through it, so airspeed tends to fall in the last few hundred feet.See Friction layer, LLWSGeneral usage
Mixed layer: The layer near the ground that daytime heating stirs into one well-mixed slab, often a few thousand feet deep in the afternoon and deeper in summer. Its top is often marked by cumulus bases or a haze top.Why it matters. Gusts at the surface come from wind mixed down from the top of this layer, and the bumps in it are thermals.See PBL, Thermals, GustGeneral usage
Gradient wind: The geostrophic wind adjusted for curved flow: a little weaker than geostrophic around lows and a little stronger around highs, for the same pressure gradient.See Geostrophic wind, Pressure gradientGeneral usage
Jet core: The narrow band at the centre of the jet stream where wind is fastest.Why it matters. Clear-air turbulence is often worst on the cold (usually poleward) side of the core and just above or below it, where shear is greatest.See Jet stream, Jet streak, Clear-air turbulence · On the site: ModelMaps: Jet stream coreGeneral usage
Shear vector: The vector difference between the wind at two heights: both speed and direction count. A 10 KT wind from 140° under a 45 KT wind from 220° at 1,000 ft is a shear of about 40 KT, far more than the 35 KT speed difference suggests.Why it matters. Wind-shear guidelines and WS groups are written in terms of the vector, not the speed change.See WS group, LLWS, Hodograph · Learn: Low-level wind shear and the nocturnal jetGeneral usage
WS group guidelines: The values a forecaster weighs before adding a low-level wind shear group to a TAF: a shear vector of more than about 25 KT within 500 ft, 40 KT within 1,000 ft or 50 KT within 1,500 ft of the ground, or a pilot report of a 20 KT airspeed change. For non-convective shear only, with the layer height no more than 1,500 ft.See WS group, Shear vector, LLWS · Learn: Low-level wind shear and the nocturnal jetGeneral usage
Frontal shear: Wind shear across a shallow frontal surface: cold air with one wind under warm air with another, often a near reversal of direction within a few hundred feet. Strongest with shallow, fast-moving cold fronts and with arctic air dammed under a Pacific flow.See Front, Shear vector, LLWS, Cold-air damming · Learn: Low-level wind shear and the nocturnal jetGeneral usage
Trapped lee wave: A mountain wave whose energy is held in a low, stable layer because the wind increases and the stability decreases with height. The waves travel far downwind as a train of crests a few to a few tens of kilometres apart, with rotors beneath and lenticular cloud above.Why it matters. The Scorer parameter falling with height is the signature; the Alberta foothills and the Okanagan produce them often.See Scorer parameter, Lee wave, Rotor, ACSL · Learn: Icing and turbulence in depth: SLD, stratocumulus, waves and CATGeneral usage
Tailwind component: The part of the wind that blows along the aircraft's track, from behind. A negative tailwind component is a headwind. The route tool averages it along the route, weighted by distance.Why it matters. It adds directly to true airspeed to give ground speed, so it sets time en route and fuel. At jet levels it can exceed 100 kt and decide which level, or which route, is best.See Headwind and tailwind component, Ground speed, CrosswindGeneral usage
Vertical wind shear (CAT clue)kt per 1000 ft: How much the wind (speed and direction together) changes per 1000 ft of height. On the flight-level winds page it is the layer average between the flight levels above and below the one you pick.Why it matters. Strong vertical shear, about 6 kt per 1000 ft or more, is the classic ingredient of clear-air turbulence: it can make the air overturn in Kelvin-Helmholtz waves. A layer average hides thin sharp layers, so treat it as a pointer and check the turbulence charts.See Clear-air turbulence, Wind shear, Kelvin–Helmholtz waves, Light, moderate, severe, extreme · On the site: Flight-level winds, ModelMaps turbulenceFAA AC 00-30C; Endlich (1964)

Terrain and regional effects

Katabatic, drainage wind: Cold, dense air flowing downhill at night or off ice and snowfields. In valleys it drains down-valley and pools in basins.Why it matters. Drainage winds decide where fog and frost collect and can give a light tailwind on one runway while the other end is calm.See Valley and slope winds, Cold pool, Valley fogGeneral usage
Valley and slope winds: On sunny days, heated slopes draw air up-slope and up-valley (anabatic flow); at night the flow reverses down-slope and down-valley.Why it matters. In mountain valleys the local wind can be the opposite of the wind in the forecast for the free air above, and it changes on a daily clock.See Katabatic, drainage wind, Daily wind cycle · Learn: Mountain waves, rotors and downslope windsGeneral usage
Gap wind: Wind squeezed and sped up through a gap in a mountain barrier, such as a pass, inlet or valley, when pressure is higher on one side.Why it matters. Wind at a gap exit can be twice what it is nearby, with strong turbulence and shear where it fans out.See Channelling, Arctic outflow, Squamish windGeneral usage
Channelling: Terrain forcing the wind to blow along a valley or strait, whatever the direction of the wind above.Why it matters. In valleys such as the Fraser Canyon, Okanagan or Bow, the surface wind follows the valley; forecasts of the free-air wind must be translated to it.See Gap wind, Valley and slope windsGeneral usage
Squamish wind: A local BC name for a strong, cold outflow wind blowing down a coastal inlet or fjord during an arctic outflow.See Arctic outflow, Gap windGeneral usage
Sea breeze, land breeze: On sunny days, land heats faster than water and cooler air from the sea moves inland in the afternoon; at night the flow can reverse as a weak land breeze.Why it matters. A sea breeze front can bring a wind shift, a drop in temperature and marine stratus or fog onto coastal airports in the evening.See Lake breeze, Sea fog, marine layer, Wind shiftGeneral usage
Lake breeze: The same as a sea breeze but off a large lake. Lake-breeze boundaries can set off afternoon thunderstorms where they meet other boundaries.See Sea breeze, land breeze, Outflow, gust frontGeneral usage
Cold pool: A layer of cold air trapped in a valley or basin under warmer air above, which can last for days in winter when the sun is weak and the wind is light.Why it matters. Persistent cold pools in interior BC and Yukon valleys keep fog, low stratus and pollution trapped while the mountains are in sunshine.See Inversion, Valley fog, Inversion stratus · Learn: Prairie inversions, chinooks and upslopeGeneral usage
Inversion stratus: A grey deck of stratus or stratocumulus trapped under a winter inversion in valleys or over the Prairies, often for days, sometimes with light snow or freezing drizzle.Why it matters. Models often clear it too soon. The ceiling may be low and icing in it is common, while just above the top it's sunny.See Cold pool, Inversion, FZDZ environment · Learn: Prairie inversions, chinooks and upslopeGeneral usage
Alberta upslope: Easterly or northeasterly flow on the Alberta plains rising toward the foothills, since the ground climbs thousands of feet from Saskatchewan to the Rockies. The lifting cools the air and makes low cloud, drizzle, fog or heavy snow.Why it matters. Upslope events bring some of the worst ceilings and heaviest spring snowstorms to Calgary and the foothills, often behind a cold front.See Upslope fog, Orographic cloud, Arctic front · Learn: Prairie inversions, chinooks and upslopeGeneral usage
Chinook arch: A band of cloud parallel to the Rockies with a clear gap of blue sky below it, seen from Alberta during a chinook. It's wave cloud formed in the flow over the mountains.Why it matters. It means strong westerly flow aloft and mountain waves: expect turbulence near and east of the foothills.See Chinook, Mountain wave, ACSL · Learn: Prairie inversions, chinooks and upslopeGeneral usage
Foehn: The general name for a warm, dry wind coming down the lee side of mountains. Air that loses moisture rising on the windward side, or that comes down from aloft, warms as it sinks. The chinook is Canada's best known.See Chinook, Rain shadow, Downslope windstormGeneral usage
Rain shadow: The drier area downwind of a mountain range, where sinking air warms and clouds break up.Why it matters. Victoria and the Okanagan sit in rain shadows; weather there can be much better than at airports facing the onshore flow.See Foehn, Orographic cloudGeneral usage
Orographic lift: Air forced upward by terrain. It can make cloud and precipitation, or set off thunderstorms if the air is unstable.See Orographic cloud, Alberta upslope, Rain shadowGeneral usage
Mountain obscuration: Cloud, precipitation or fog hiding mountain tops, ridges or passes.Why it matters. Valleys can be VFR while the passes you need are closed by cloud. The GFA and weather cameras are the best guides.See Orographic cloud, Weather camera, Mountain pass weatherGeneral usage
Mountain pass weather: The weather at the high point of a route through the mountains, which is often worse than at the airports at either end: lower cloud, stronger wind and more snow.See Mountain obscuration, Gap wind, Weather cameraGeneral usage
Lee trough, lee low: A trough or low that forms on the east side of the Rockies as air descends the slopes and stretches vertically. Lows that develop there can deepen as they move east.Why it matters. Many Prairie storms start this way, including the Alberta clipper.See Trough, ridge, Alberta clipper, ChinookGeneral usage
Alberta clipper: A fast-moving low that forms east of the Rockies in Alberta and races southeast across the Prairies, usually in winter. It brings strong wind, blowing snow and a sharp cold front, but often only light snowfall.See Lee trough, lee low, BLSN, DRSN, Arctic frontGeneral usage
Colorado low: A low that forms east of the Rockies in the US and tracks northeast. It can bring heavy snow, freezing rain and blizzards to southern Manitoba, Ontario and Quebec.See Lee trough, lee low, BlizzardGeneral usage
Atmospheric river: A long, narrow corridor of strong water-vapour transport in the lower atmosphere, usually just ahead of a Pacific cold front: typically over 2000 km long and a few hundred kilometres wide, with integrated water vapour transport (IVT) of 250 kg m⁻¹ s⁻¹ or more. One that reaches back toward Hawaii and hits southern BC is sometimes called the Pineapple Express.Why it matters. It brings heavy rain, high freezing levels and flooding to the coast and mountains, and snow at higher levels. Strength is ranked AR1 to AR5 on the AR scale.See Integrated water vapour transport, AR scale, Precipitable water, Orographic lift, Heavy rain, Pineapple Express · On the site: Atmospheric rivers in BC, ModelMaps: Precipitable waterGeneral usage
Barrier jet: A band of strong low-level wind that runs along a mountain range when stable air is blocked and turns to flow parallel to it, as along BC's Coast Mountains ahead of Pacific storms.See Froude number, Gap wind, Low-level jetGeneral usage
Dryline: A boundary between moist air and very dry air, without much temperature difference. On the Alberta plains in summer one can form near the foothills.Why it matters. Thunderstorms often start along it in the afternoon.See Outflow, gust front, CAPE, Hail alleyGeneral usage
Hail alley: The Alberta foothills and plains between about Calgary and Red Deer, one of the most hail-prone areas in Canada, with storms in June to August.See GR, GS, Dryline, Hail size forecast · Learn: Thunderstorms and convective parametersGeneral usage
Terrain-induced wind shear: Shear and turbulence caused by wind blowing over or around hills, buildings or valley walls near an airport.Why it matters. At mountain and coastal airports it can be worse with certain wind directions; local knowledge and PIREPs matter.See Mechanical turbulence, LLWS, RotorGeneral usage
Sea-surface temperature: The temperature of the top layer of the ocean or a lake.Why it matters. Cold water under moist air means advection fog; warm open water under arctic air means steam fog, lake-effect snow and showers.See Sea fog, marine layer, Steam fog, sea smoke, Lake-effect snowGeneral usage
Freeze-up season: The weeks in autumn when lakes, rivers and coastal waters in the North are freezing over. Cold air over the remaining open water gives steam fog, stratus and snow showers downwind until the ice closes.See Steam fog, sea smoke, Arctic stratus, Lake-effect snow · Learn: Forecasting fog and stratus: radiation, advection and upslopeGeneral usage
Cold-air damming: Cold, dense air held against the east side of a mountain range by its own weight and by the terrain, so that it stays in place while warmer air flows over the top. On the Prairies the Rockies dam arctic air on the Alberta plains; the chinook rides over it.Why it matters. The dammed layer is where upslope stratus, freezing drizzle and freezing rain live, and it can take days to erode.See Arctic front, Upslope fog, Chinook, Frontal inversion · Learn: Arctic fronts, lee troughs and the TROWAL on the PrairiesGeneral usage
AR landfall: When and where an atmospheric river reaches the coast. On this site the landfall IVT is read from the ocean cells just off the open coast, every 0.25° of latitude from 32 to 59° N; the BC coast is 48.3 to 54.7° N, from Juan de Fuca Strait to Dixon Entrance.Why it matters. The hours around landfall bring the heaviest rain, the highest freezing levels and the strongest low-level winds to the coast and the Coast Mountains; the plume then weakens as the mountains wring out its moisture.See Atmospheric river, Integrated water vapour transport, Orographic lift, Rain shadow · On the site: Atmospheric rivers in BCGeneral usage
Pineapple Express: A popular name for an atmospheric river whose plume can be traced back toward the subtropical Pacific near Hawaii and that reaches the Pacific Northwest or BC. It is one kind of atmospheric river, not another name for all of them.Why it matters. These plumes are usually warm, so they bring very high freezing levels, rain on snow and flooding rather than mountain snow.See Atmospheric river, Integrated water vapour transport, FZLVLGeneral usage
Fetch: The distance the wind blows over open water. For lake-effect snow, a longer fetch along a lake gives the air more time to pick up heat and moisture: bands usually need on the order of 100 km or more of open water, so wind directions along a lake's long axis give the heaviest bands.See Lake-effect snow, ΔT (lake minus 850 hPa), Snow squall · On the site: Lake-effect snowGeneral usage
ΔT (lake minus 850 hPa)Lake–air temperature difference: The lake surface temperature minus the air temperature at 850 hPa (about 1,500 m). At about 13 °C or more the air over the lake becomes unstable enough for lake-effect snow; larger differences mean deeper convection and heavier bands.Why it matters. ΔT is the single most-used lake-effect ingredient. It falls as lakes cool through the winter and once a lake freezes over, which ends the season for that lake.See Lake-effect snow, Fetch, Stable, unstable, neutral, 850 hPa chart · On the site: Lake-effect snowGeneral usage (Holroyd 1971; Niziol 1987)

Thunderstorms and convection

CAPEConvective available potential energy: The energy a rising parcel of air could gain from being warmer than its surroundings. More CAPE allows stronger updrafts; it says nothing about whether storms will actually start.General usage
MUCAPEMost-unstable CAPE: CAPE for the most unstable parcel in the lowest few hundred hPa, which catches storms based above a cool surface layer.General usage
CINConvective inhibition: The energy needed to push air up to where it can rise on its own: a lid. A large CIN can hold storms off even with lots of CAPE.General usage
DCAPEDowndraft CAPE: How strongly rain-cooled air could sink and spread out from a storm. Higher values mean stronger outflow gusts.General usage
Lifted indexLI: How much cooler (positive) or warmer (negative) a lifted surface parcel is than the air around it at 500 hPa. Negative values mean unstable air; below about −6 is very unstable.General usage
Total TotalsTT: An old stability index from 850 and 500 hPa temperature and dew point. Roughly, mid 40s suggests some thunderstorms and mid 50s or more strong ones; thresholds vary by region.General usage
Bulk shear0-6 km shear: The difference between the wind near the ground and the wind about 6 km up. Around 35 to 40 KT or more lets storms organise into long-lived and supercell storms.General usage
SRHStorm-relative helicity: How much the low-level wind turns and strengthens with height relative to a moving storm. High values favour rotating storms and tornadoes.General usage
STPSignificant tornado parameter: A combined index of instability, shear, helicity and cloud base height. Values above 1 point to environments that have supported strong tornadoes in US studies.General usage
SHIPSignificant hail parameter: A combined index for hail 2 inches (5 cm) or larger. Values above 1 suggest a favourable environment.General usage
Wet-bulb zero: The height where the wet-bulb temperature reaches 0 °C. Lower values help hail reach the ground without melting.General usage
Miller method: A 1970s US Air Force checklist (TR-200) for forecasting severe storms from air-mass type, synoptic pattern and indices. Our charts read it from the model; some rows are our interpretation.General usage · This site
ReflectivitydBZ: How much radar energy precipitation sends back. Roughly 20 dBZ is light rain or snow, 40 heavy rain and 50 or more often hail or very heavy rain.General usage
Composite reflectivity: The strongest reflectivity anywhere in the column above each point, so the strongest cores show even if they are aloft.General usage
Outflow, gust front: Cold air rushing out from under a storm. Its leading edge brings a sudden wind shift and gusts, sometimes many kilometres from the storm.General usage
Microburst: A small, intense downdraft under a storm or shower that spreads out at the ground, giving dangerous wind shear for aircraft close to the ground.General usage
Overshooting top: A bubble on top of a thunderstorm anvil where a strong updraft punches above it. On infrared images it shows as a small, very cold spot.General usage
Anvil: The flat, spreading ice cloud at the top of a thunderstorm, blown downwind by the upper winds.General usage
EMBDEmbedded: Convective cloud hidden inside a wider layer of cloud, so pilots in cloud can't see it coming.General usage
P(TS)Thunderstorm probability: Our chance of thunder near a point, calibrated against lightning. It goes into TAF-style text only as PROB groups.This site
Thunderstorm ingredients: Three things are needed: moisture in the low levels, instability (air that keeps rising once lifted) and something to start the lift, such as heating, a front, terrain or an outflow boundary. Wind shear then decides how organised the storms get.Why it matters. Checking each ingredient separately is the quickest way to judge whether a convective forecast makes sense.See CAPE, Trigger, lift, Bulk shear · Learn: Thunderstorms and convective parametersGeneral usage
Thunderstorm life cycle: A single storm cell goes through a growing stage (cumulus, all updraft), a mature stage (updraft and downdraft side by side, heaviest rain, hail and lightning) and a dissipating stage (downdraft takes over). A cell typically lasts 30 to 60 minutes.Why it matters. Gust fronts and microbursts come in the mature and dissipating stages, often after the storm looks like it's weakening.See CB, TCU, ACC, CU, Outflow, gust front, Microburst · Learn: Thunderstorms and convective parametersGeneral usage
Single-cell stormPulse or air-mass storm: An isolated storm that grows and dies in about an hour, usually in weak wind shear, often from afternoon heating.Why it matters. Short-lived but can still give a brief microburst, small hail and heavy rain.See Thunderstorm life cycle, Multicell storm · Learn: Thunderstorms and convective parametersGeneral usage
Multicell storm: A cluster of cells at different stages, where each cell's outflow sets off new cells, usually on one side. The cluster can last for hours though each cell is brief.See Single-cell storm, Outflow, gust front, Squall line · Learn: Thunderstorms and convective parametersGeneral usage
Supercell: A long-lived storm with a deep rotating updraft (a mesocyclone), needing strong wind shear. Supercells make most large hail and strong tornadoes.Why it matters. On the Prairies in summer, supercells can last hours and travel a long way. Avoid them by a wide margin.See Bulk shear, SRH, Wall cloud, Hook echo · Learn: Thunderstorms and convective parametersGeneral usage
Squall line: A line of thunderstorms, often ahead of or along a cold front, with a continuous gust front at its leading edge.Why it matters. A line can be hard to get around, and the wind shift and gusts along it are strong and sudden.See Shelf cloud, arcus, Outflow, gust front, Bow echo · Learn: Thunderstorms and convective parametersGeneral usage
MCSMesoscale convective system: A large organised group of thunderstorms, hundreds of kilometres across, that can last through the night with a wide area of rain behind the leading storms.See Squall line, Nighttime thunderstormsGeneral usage
Derecho: A long-lived, widespread windstorm from a fast-moving line of thunderstorms, causing damaging straight-line winds along a path hundreds of kilometres long.See Bow echo, Squall lineGeneral usage
Bow echo: A line of storms on radar that bulges forward into a bow shape, pushed by a strong rear-inflow jet. The strongest winds are near the tip of the bow.See Squall line, Derecho, ReflectivityGeneral usage
Hook echo: A hook-shaped appendage on the radar image of a supercell, where rain wraps around the rotating updraft. It is a classic tornado signature.See Supercell, ReflectivityGeneral usage
Severe thunderstorm: In Canada, a storm that produces one or more of: hail 2 cm or larger, wind gusts of 90 km/h or more, very heavy rain, or a tornado. These are ECCC's public warning criteria.Why it matters. Severe storms are a small share of all storms but cause most of the damage. Our severe hail and wind probabilities use the same hail and gust thresholds.See ECCC public alerts, GR, GS, Tornado · On the site: ModelMaps: Severe hail, wind and tornado probability · Learn: Thunderstorms and convective parametersGeneral usage
Air parcel: An imaginary blob of air followed as it rises or sinks, cooling or warming without mixing with its surroundings. Comparing its temperature with the air around it shows whether it would keep rising.See LCL, LFC, CAPE · Learn: Reading a soundingGeneral usage
LCLLifting condensation level: The height at which a rising parcel cools to its dew point and cloud forms. It's a good estimate of the base of cumulus cloud.Why it matters. A low LCL means low cloud bases. A rough rule: every 1 °C of spread between temperature and dew point at the surface puts the base about 400 ft higher.See Air parcel, LFC, Spread · Learn: Reading a soundingGeneral usage
LFCLevel of free convection: The height above which a lifted parcel becomes warmer than its surroundings and rises on its own.See LCL, EL, CIN · Learn: Reading a soundingGeneral usage
ELEquilibrium level: The height where a rising parcel becomes as cold as its surroundings again, roughly where a storm's anvil spreads out. Strong updrafts overshoot it.See LFC, Anvil, Overshooting top · Learn: Reading a soundingGeneral usage
Cap, lid: A warm, stable layer aloft that stops surface parcels from rising, measured by CIN. A strong cap can hold off storms all day; if it breaks, the storms that do form can be explosive.See CIN, Inversion, Trigger, lift · Learn: Thunderstorms and convective parametersGeneral usage
Trigger, lift: What starts air rising to its level of free convection: daytime heating, fronts, troughs, terrain, sea or lake breezes, drylines and outflow boundaries from earlier storms.Why it matters. Plenty of CAPE without a trigger can mean a dry, sunny day. Find the trigger to find where and when storms start.See Thunderstorm ingredients, Cap, lid, Outflow, gust front · Learn: Thunderstorms and convective parametersGeneral usage
Elevated thunderstorm: A storm fed by unstable air above a cool, stable surface layer, often north of a warm front or at night. The surface air doesn't take part.Why it matters. Surface-based CAPE can be zero while elevated storms rumble overhead. Most-unstable CAPE catches them.See MUCAPE, Nighttime thunderstorms, Warm frontGeneral usage
Nighttime thunderstorms: Storms that form or keep going after dark, often elevated and fed by a low-level jet bringing in moisture.See Elevated thunderstorm, Nocturnal low-level jet, MCSGeneral usage
Storm motion: Storms move roughly with the mean wind through their depth, but new cells forming on one side shift the system. Supercells often move to the right of the mean wind.See Supercell, Multicell storm, HodographGeneral usage
Storm avoidance: Common guidance is to stay well clear of strong thunderstorms, often quoted as 20 NM for severe ones, and never to fly under an anvil or between closely spaced cells.Why it matters. Hail can fall out of the anvil miles from the core, and gust fronts reach far ahead of the rain.See Anvil, Outflow, gust front, GR, GS · Learn: Thunderstorms and convective parametersGeneral usage
K index: A simple index from 850, 700 and 500 hPa temperatures and dew points that combines lapse rate with mid-level moisture. Higher values mean more chance of air-mass thunderstorms.See Total Totals, Lifted index, Stable, unstable, neutralGeneral usage
Showalter index: Like the lifted index, but lifting a parcel from 850 hPa instead of the surface. Negative values mean instability above the surface layer.See Lifted index, Elevated thunderstormGeneral usage
Hail size forecast: An estimate of the largest hail a storm could make, from updraft strength and the temperatures aloft. Ours uses the Fawbush–Miller approach on model soundings.See GR, GS, Severe thunderstorm, Wet-bulb zero · On the site: ModelMaps: Hail size (Fawbush-Miller)This site
Anvil hazards: The anvil spreading downwind from a storm top can hold hail, turbulence and lightning, and mammatus under it marks sinking, rough air.See Anvil, Mammatus, Storm avoidanceGeneral usage
Towering cumulus growth: Cumulus that keep building upward with hard, sharp tops are on their way to becoming thunderstorms. When the top turns soft and fibrous, it has frozen into ice: the cloud has become a cumulonimbus.Why it matters. A rapidly growing TCU can go from bumpy to lightning in 15 to 20 minutes.See CB, TCU, ACC, CU, Cloud-top cooling, Cloud-top phaseGeneral usage
Convection: Vertical motion driven by buoyancy: warm air rising and cooler air sinking. In aviation the word usually means cumulus and thunderstorms.See Thermals, CAPE, CB, TCU, ACC, CUGeneral usage
Mesoscale boundary: A narrow zone where air properties change sharply over a small distance: outflow boundaries, sea and lake breezes, drylines and old fronts.Why it matters. New storms often start where boundaries meet or where a boundary hits terrain.See Outflow, gust front, Sea breeze, land breeze, DrylineGeneral usage
Convective temperature: The surface temperature at which the afternoon mixed layer reaches the level where a lifted parcel becomes buoyant, so that convection starts without any other trigger. Read from the sounding by following a dry adiabat down from the point where the moist adiabat clears the cap.Why it matters. The hour the forecast temperature reaches it is a first estimate of thunderstorm initiation time.See CIN, Cap, lid, LCL, Trigger, lift · Learn: Convective initiation and severe parameters for aviationGeneral usage
Outflow gust: The surface gust produced by a thunderstorm's downdraft spreading out at the ground. Its strength grows with the dryness of the air the rain falls through (evaporative cooling), the depth of that dry layer and the storm's forward speed.Why it matters. A 50 KT gust from a modest storm is common on the Prairies when the sub-cloud layer is deep and dry; the storm need not be severe on radar.See Microburst, DCAPE, Evaporative cooling, Gust spread · Learn: Convective initiation and severe parameters for aviationGeneral usage

Upper air and fronts

hPaHectopascal: The unit for pressure on weather maps, equal to the millibar. Standard sea-level pressure is 1013.25 hPa.General usage
850, 700, 500, 250 hPa: Standard upper-air levels, roughly 5,000, 10,000, 18,000 and 34,000 ft above sea level. Heights vary with temperature: lower in cold air, higher in warm air.General usage
Height (geopotential): How high a pressure level is, in decametres on our maps (dam: tens of metres). 552 dam at 500 hPa means 5,520 m.General usage
Thickness: The depth between two pressure levels, usually 1000 and 500 hPa. Thicker means warmer air. Near 540 dam is a rough rain/snow line, not a rule.General usage
Vorticity: Spin in the air. On 500 hPa maps, areas of high (cyclonic) vorticity moving in often mark upper disturbances that help cloud and precipitation.General usage
Trough, ridge: A trough is an elongated area of lower pressure or height, often with cloud and unsettled weather; a ridge is the opposite, usually with fair weather.General usage
Low, high: Centres of low and high pressure. In the northern hemisphere wind blows counter-clockwise around a low and clockwise around a high.General usage
Front: The boundary between two air masses. A cold front brings colder air, often with a sharp wind shift and showers; a warm front brings warmer air with widespread low cloud and precipitation ahead of it.General usage
Occluded front, TROWAL: Where a cold front catches up with a warm front and lifts the warm air off the ground. A TROWAL (trough of warm air aloft) is the Canadian term for the warm tongue aloft, often with a band of steady precipitation.General usage
Frontogenesis: A front strengthening as the wind squeezes temperature contrasts together. It forces air to rise and can give narrow bands of heavy precipitation.General usage
CSIConditional symmetric instability: A slantwise instability that lets air rise along a sloping path when lifted. With frontogenesis it often gives heavy snow bands. Not the CSI verification score.General usage
EPVEquivalent potential vorticity: Negative values in a saturated layer mean the air is open to CSI or upright convection.General usage
Precipitable waterPW: All the water vapour in the column above a point, as a depth of liquid if it all fell out. Higher values mean more fuel for heavy precipitation.General usage
PV, 2 PVUPotential vorticity: A quantity that is high in the stratosphere. The 2 PVU surface is a common way to mark the dynamic tropopause.General usage
Air mass: A large body of air with similar temperature and moisture, such as cold arctic air or mild Pacific air.General usage
Snow-to-liquid ratio: How deep the snow is compared with its melted water. Our snowfall maps assume 10:1 (10 cm of snow per 10 mm of water); real snow ranges from about 5:1 (wet) to 20:1 or more (fluffy).General usage · This site
QPFPrecipitation amount: Quantitative precipitation forecast: how much rain or melted snow is expected, in millimetres.General usage
Cold front: The leading edge of colder air pushing under warmer air. The slope is steep, so weather along it is often narrow and showery: a line of cumulus or thunderstorms, a wind shift, gusts and a pressure rise behind.See Front, FROPA, Squall lineGeneral usage
Warm front: The leading edge of warmer air riding up over colder air. The slope is gentle, so cloud lowers and thickens over hundreds of kilometres ahead of it: cirrus, then altostratus, then nimbostratus with steady precipitation.Why it matters. In winter, the shallow cold air under a warm front is where freezing rain and ice pellets happen, and low ceilings can last for hours.See Front, Warm nose, NSGeneral usage
Stationary front: A front that has stalled, with neither air mass pushing the other. Weather along it can persist for a day or more.See Front, Cold front, Warm frontGeneral usage
Isobar: A line on a surface map joining places with the same sea-level pressure, usually every 4 hPa. Closely spaced isobars mean strong wind.See MSLP, Geostrophic wind, Height contour · On the site: ModelMaps: Pressure, precipitation and windGeneral usage
Height contour: On upper-air charts, a line joining places where a pressure level (such as 500 hPa) is at the same height, labelled in decametres. Upper winds blow along the contours.See Height (geopotential), Isobar, Geostrophic wind · On the site: ModelMaps: 500 hPa height and vorticityGeneral usage
Isotherm: A line joining places with the same temperature. On an 850 hPa chart, packed isotherms mark fronts and the 0 °C isotherm gives a first guess at the rain–snow line.See 850 hPa chart, Front, Rain–snow lineGeneral usage
850 hPa chart: The map for about 5,000 ft ASL, close to the top of the friction layer in most places and below ground level over the high Rockies. Used for low-level temperature, moisture, fronts and the low-level jet.See 850, 700, 500, 250 hPa, Isotherm, Theta-w · On the site: ModelMaps: 850 hPa temperatureGeneral usage
500 hPa chart: The map for about 18,000 ft, near the middle of the atmosphere by mass. Troughs, ridges and vorticity there steer surface weather systems.Why it matters. It's the forecaster's first look at the big picture: where the troughs are, how fast they move and where the lift will be.See Trough, ridge, Vorticity, Short-wave trough · On the site: ModelMaps: 500 hPa height and vorticityGeneral usage
Short-wave trough: A small, fast-moving trough embedded in the larger flow, often a few hundred to a thousand kilometres across. Lift ahead of it brings cloud and precipitation.Why it matters. Short waves are the triggers for many weather events, but models can place them a few hours off, which matters for timing.See Trough, ridge, Vorticity, Long wavesGeneral usage
Long wavesRossby waves: The few big, slow-moving troughs and ridges that wrap around the hemisphere in the westerlies. They set the weather pattern for days to weeks.See Short-wave trough, Blocking pattern, Upper ridgeGeneral usage
Upper ridge: A northward bulge of high heights in the upper flow. Under it, air sinks and the weather is usually settled, though inversions, fog and smoke can be trapped below.See Trough, ridge, Subsidence, Blocking patternGeneral usage
Cut-off low: An upper low that has separated from the main westerly flow and drifts slowly or stalls, sometimes for days.Why it matters. Models struggle with when it will move, and it can give long spells of cloud and precipitation, or thunderstorms, over the same area.See Blocking pattern, Trough, ridgeGeneral usage
Blocking pattern: A large, persistent ridge or low (an omega block, for example) that stops the usual west-to-east movement of weather systems.Why it matters. Blocks bring long spells of the same weather: prolonged heat and smoke in summer, or a deep arctic outbreak in winter.See Upper ridge, Cut-off low, Long wavesGeneral usage
Cyclogenesis: The birth or deepening of a low-pressure system, usually where an upper trough moves over a low-level temperature contrast.See Lee trough, lee low, Low, high, Baroclinic zoneGeneral usage
Warm and cold advection: Warm advection is wind carrying warmer air into an area; cold advection carries colder air in. On a chart, wind blowing across isotherms from warm to cold shows warm advection.Why it matters. Warm advection tends to give rising air, layered cloud and steady precipitation; cold advection gives sinking, clearing or showery, gusty weather.See Veering, backing, Isotherm, FrontGeneral usage
Vorticity advection: Wind carrying spin into an area. Positive vorticity advection ahead of an upper trough is linked to rising air and surface pressure falls.See Vorticity, Short-wave trough, Vertical motion, omegaGeneral usage
Divergence, convergence: Air spreading out (divergence) or piling in (convergence). Divergence aloft over convergence at the surface makes air rise between them.See Vertical motion, omega, Jet streakGeneral usage
Vertical motion, omega: How fast air rises or sinks, usually given on model charts in pressure units per second (omega), where negative means rising.Why it matters. Rising motion through a moist layer means cloud and precipitation. It's what most other upper-air charts are trying to diagnose.See Divergence, convergence, Vorticity advection, SubsidenceGeneral usage
Jet streak: A stretch of the jet stream where the wind is fastest. Air speeds up entering it and slows leaving it, which makes rising motion in certain quadrants (the right entrance and left exit, in the usual case).See Jet stream, Divergence, convergence, Isotach · On the site: ModelMaps: 250 hPa wind (jet stream)General usage
Polar and subtropical jets: The polar jet runs along the boundary between polar and mid-latitude air, near 30,000 to 35,000 ft, and is the one that affects Canada most. The subtropical jet is higher and further south.See Jet stream, TropopauseGeneral usage
Polar vortex: The large circulation of cold air around the pole. When a lobe of it swings south, Canada gets a long, deep cold spell.See Blocking pattern, Arctic frontGeneral usage
Stratosphere: The layer above the tropopause where temperature stops falling with height and starts to rise. It's very dry and stable.See Tropopause, Overshooting topGeneral usage
Lapse rate: How fast temperature falls with height. The standard atmosphere averages about 2 °C per 1,000 ft; real air can be much steeper, or even warm with height in an inversion.Why it matters. Steep lapse rates mean unstable, bumpy air and showers; shallow ones mean stable, layered cloud and smooth air.See Dry adiabatic lapse rate, Moist adiabatic lapse rate, Stable, unstable, neutral, ISA · Learn: Reading a soundingGeneral usage
Dry adiabatic lapse rate: The rate an unsaturated parcel cools as it rises, about 3 °C per 1,000 ft (9.8 °C per km).See Lapse rate, Moist adiabatic lapse rate, Adiabatic · Learn: Reading a soundingGeneral usage
Moist adiabatic lapse rate: The slower rate a saturated parcel cools as it rises, because condensation releases heat. It's about 1.5 °C per 1,000 ft in warm, moist air and closer to the dry rate in very cold air.See Dry adiabatic lapse rate, Latent heat, Lapse rate · Learn: Reading a soundingGeneral usage
Adiabatic: A change with no heat added or removed. Rising air expands and cools adiabatically; sinking air is compressed and warms.See Dry adiabatic lapse rate, Subsidence, FoehnGeneral usage
Stable, unstable, neutral: Stable air pushes a displaced parcel back where it came from; unstable air lets it keep going; neutral air leaves it where it is. Stability depends on the lapse rate compared with the dry and moist adiabatic rates.Why it matters. Stable air: layered cloud, poor visibility in haze and fog, smooth flight, mountain waves. Unstable air: cumulus, showers, good visibility outside them, bumpy flight.See Lapse rate, Conditional instability, Inversion · Learn: Reading a soundingGeneral usage
Conditional instability: A layer that is stable for dry air but unstable for saturated air: its lapse rate lies between the moist and dry adiabatic rates. Lift it to saturation and it can take off.See Stable, unstable, neutral, LFC, CAPE · Learn: Reading a soundingGeneral usage
Potential temperature, θ: The temperature air would have if brought down dry-adiabatically to 1000 hPa. It stays the same for dry air moving up or down, so it's a label for an air mass.See Theta-w, Equivalent potential temperature, θe, AdiabaticGeneral usage
Equivalent potential temperature, θe: Like potential temperature, but counting the heat that would be released if all the moisture condensed. High θe means warm, moist air with storm fuel.See Potential temperature, θ, Theta-w, CAPEGeneral usage
Latent heat: Heat absorbed when water evaporates or ice melts, and released when vapour condenses or water freezes.Why it matters. It powers thunderstorms, slows the cooling of saturated air, and explains why melting snow can cool a layer to 0 °C and hold it there.See Moist adiabatic lapse rate, Wet-bulb temperature, Melting layerGeneral usage
Mixing ratio: The mass of water vapour per kilogram of dry air, in grams per kilogram. It stays the same as unsaturated air rises or sinks, unlike relative humidity.See Dew point, RH, Precipitable water · Learn: Reading a soundingGeneral usage
Subsidence: Large-scale sinking of air, usually under a ridge or high. The sinking air warms and dries, often forming a subsidence inversion above the boundary layer.Why it matters. Subsidence clears skies aloft but can trap haze, smoke, fog and low stratus beneath its inversion.See Upper ridge, Inversion, AdiabaticGeneral usage
Deformation zone: An area where the flow stretches in one direction and squeezes in another, often northwest of a low, where frontal bands of snow or rain can form and sit.See Frontogenesis, Comma cloud, Wrap-around precipitationGeneral usage
Comma cloud: The comma shape of cloud around a mature low seen on satellite: the tail is the cold front, the head wraps around the north side of the low.See Dry slot, Occluded front, TROWAL · On the site: Satellite: Water vapour and Air mass RGBGeneral usage
Dry slot: A tongue of dry, sinking air wrapping into a low behind the cold front, seen as a dark band on water vapour imagery.Why it matters. Precipitation can stop suddenly when it arrives, but showers and gusty wind may follow.See Comma cloud, Water vapour imageryGeneral usage
Wrap-around precipitation: Precipitation on the north and west side of a mature low, fed by warm, moist air wrapped around it aloft (the TROWAL). On the Prairies it gives many of the heaviest snowfalls.See Occluded front, TROWAL, Deformation zone, BlizzardGeneral usage
Conveyor belts: A simple model of the main airstreams in a mid-latitude low: a warm conveyor belt rising ahead of the cold front, a cold conveyor belt flowing westward north of the warm front, and dry air descending behind.See Comma cloud, Dry slot, Warm frontGeneral usage
Baroclinic zone: An area with a strong temperature contrast, where lows can form and deepen and winds increase with height.See Front, Cyclogenesis, ThicknessGeneral usage
Coriolis effect: The apparent turning of moving air to the right in the northern hemisphere, caused by the Earth's rotation. It's why wind circles a low counter-clockwise.See Geostrophic wind, Buys Ballot's lawGeneral usage
Buys Ballot's law: In the northern hemisphere, with your back to the wind, low pressure is on your left.Why it matters. Flying toward a low with a wind from the right means drift to the left and lower heights than the altimeter shows: the old warning 'from high to low, look out below'.See Coriolis effect, Low, high, Standard pressure, 29.92General usage
Pressure gradient: How quickly pressure changes over distance. The tighter the isobars, the stronger the gradient and the wind.See Isobar, Geostrophic windGeneral usage
Thermal wind: The change of wind with height caused by a horizontal temperature contrast. Wind increases with height along the temperature contrast, with cold air on the left (northern hemisphere).Why it matters. It explains why jet streams sit above strong fronts and why winds aloft are strongest in winter.See Jet stream, Baroclinic zone, ThicknessGeneral usage
Advection: The carrying of a property, such as heat, moisture or cloud, by the wind from one place to another.See Warm and cold advection, Advection fogGeneral usage
Synoptic scale, mesoscale: Synoptic scale means weather systems about 1,000 km and larger, like highs, lows and long fronts, lasting days. Mesoscale means a few to a few hundred kilometres, like storms, sea breezes and mountain waves, lasting hours.Why it matters. Models and forecasts are much better at the synoptic scale. Most aviation hazards live at the mesoscale.See Predictability, Grid spacingGeneral usage
Air pressure: The weight of the air above a point. It falls with height, by about 1 inch of mercury (34 hPa) per 1,000 ft near sea level, which is how altimeters work.See Altimeter setting, hPa, IsobarGeneral usage
Bomb, explosive deepening: A low whose central pressure falls very fast, by roughly 24 hPa in 24 hours at mid-to-high latitudes (less further south). Common over the North Pacific and Atlantic in the cold season.Why it matters. Explosive lows bring hurricane-force wind and heavy precipitation to the BC coast and Atlantic Canada.See Cyclogenesis, Low, highGeneral usage
El Niño, La Niña: Swings in Pacific Ocean temperatures near the equator that shift jet streams worldwide. In western Canada, El Niño winters tend to be milder and La Niña winters colder and snowier, though no two are alike.See Oceanic Niño Index, Relative Oceanic Niño Index, Niño-3.4 region, Southern Oscillation Index, ENSO alert system, Climatology, Jet stream · On the site: ENSO trackerGeneral usage
Sublimation, deposition: Ice turning straight to vapour (sublimation) or vapour straight to ice (deposition), without passing through liquid.See Dew, frost, hoar frost, Ice-crystal processGeneral usage
TROWALTrough of warm air aloft: The tongue of warm, moist air lifted above the surface in an occluding low, drawn on Canadian charts as a line running from the triple point back toward the low centre. Under it the heaviest wrap-around precipitation falls.Why it matters. Prairie snowstorms with the longest duration and heaviest snow sit under the TROWAL, often north and west of the surface low, far from the fronts on the map.See Wrap-around precipitation, Occluded front, TROWAL, Theta-w, Deformation zone · Learn: Arctic fronts, lee troughs and the TROWAL on the PrairiesGeneral usage
Upper front: A front whose main temperature contrast is aloft, at 850 or 700 hPa, with little or no signature at the surface. Common when a Pacific front rides over a dammed arctic layer east of the Rockies.Why it matters. The surface map looks quiet while the cloud, icing and precipitation of a front pass overhead; wet-bulb potential temperature at 850 and 700 hPa shows it.See Front, Theta-w, Cold-air damming · Learn: Arctic fronts, lee troughs and the TROWAL on the PrairiesGeneral usage
Frontal inversion: The temperature increase with height where warm air lies on top of a colder air mass along a frontal surface. On a sounding it marks the top of the cold layer and often the warm nose that drives freezing precipitation.See Inversion, Warm nose, Cold-air damming · Learn: Arctic fronts, lee troughs and the TROWAL on the PrairiesGeneral usage
Integrated water vapour transportIVT: How much water vapour the wind is carrying sideways through a column of air, added up from the ground to high in the atmosphere. It is the column's moisture multiplied by the wind that moves it, in kilograms of vapour per second crossing each metre of width (kg m⁻¹ s⁻¹).Why it matters. IVT is the number atmospheric rivers are defined and ranked by: 250 kg m⁻¹ s⁻¹ or more marks AR conditions. High IVT aimed at mountains means heavy rain or snow, high freezing levels and low ceilings on the windward side.See Atmospheric river, AR scale, Precipitable water, Orographic lift · On the site: Atmospheric rivers in BCZhu and Newell 1998; Ralph et al. 2019
AR scaleAR1 to AR5: A five-step ranking of atmospheric rivers from CW3E at Scripps (Ralph et al. 2019). The peak IVT of the event sets the starting rank (250–500 AR1, 500–750 AR2, 750–1000 AR3, 1000–1250 AR4, 1250 or more AR5, in kg m⁻¹ s⁻¹); it moves up one step when AR conditions last 48 hours or more at a place and down one step when they last under 24 hours.Why it matters. AR1 and AR2 are mostly beneficial (water supply, snowpack); AR4 and AR5 are mostly hazardous (floods, landslides, rain on snow). The rank describes the moisture flow, not the rain at a given airport, which also depends on terrain and duration.See Integrated water vapour transport, Atmospheric river, AR object · On the site: Atmospheric rivers in BCRalph et al. 2019, BAMS
AR object: An area on a map that an automatic method has picked out as an atmospheric river: a connected patch of IVT at or above a threshold that is also long, narrow and carries moisture poleward in one coherent direction. On this site: IVT ≥ 250 kg m⁻¹ s⁻¹, at least 2000 km long (1000 km when cut by the map edge), at least twice as long as wide, with poleward transport (after Guan and Waliser 2015).Why it matters. The shape tests separate true moisture plumes from round blobs of moist tropical air or the moist core of a low, so an outline on the map really is a river-like plume.See Atmospheric river, Integrated water vapour transport, AR scale · On the site: Atmospheric rivers in BCGuan and Waliser 2015; Rutz et al. 2014
Jet axisJet core line: The line along the centre of a jet stream where the wind is strongest across the flow. MeteoFrontier draws it where the strongest wind between FL180 and FL450 reaches a peak across the flow and is at least 80 kt.Why it matters. The strongest tailwinds (or headwinds) lie along the axis. Clear-air turbulence is most common just beside it, on the cold (poleward) side and under it near the tropopause fold, and in the entrance and exit regions.See Jet stream, Jet core, Jet streak, Clear-air turbulence, Isotach · On the site: Flight-level windsGeneral usage; method after Berry, Reeder and Jakob (2007)
Sudden stratospheric warmingSSW: A breakdown of the winter polar vortex in which the air 30 km above the Arctic warms by 30–50 °C in a few days. A major SSW is declared when the zonal-mean wind at 60°N, 10 hPa turns from westerly to easterly between November and March.Why it matters. In about two of three major events the weak-vortex state works down to the troposphere within 2–8 weeks, shifts the jet stream south and raises the odds of cold outbreaks in parts of North America (often eastern Canada and the Great Lakes), Europe and Asia.See Polar vortex, Zonal-mean zonal wind, Vortex split, Vortex displacement, Stratosphere–troposphere coupling · On the site: Polar vortex and teleconnectionsCharlton and Polvani 2007; Baldwin and Dunkerton 2001
Zonal-mean zonal windu at 60°N, 10 hPa: The west-to-east wind component averaged all the way around a latitude circle. At 60°N and 10 hPa (about 31 km) it measures the strength of the stratospheric polar vortex.Why it matters. It is the number that defines a major sudden stratospheric warming: below zero (easterly) in winter means the vortex has collapsed.See Sudden stratospheric warming, Polar vortex · On the site: Polar vortex and teleconnectionsCharlton and Polvani 2007
Polar-cap temperature: The area-weighted average temperature north of 60°N on a pressure level, often 10 or 50 hPa.Why it matters. A rapid rise of 20 °C or more at 10 hPa is the 'warming' in a sudden stratospheric warming; a very cold polar cap goes with a strong vortex.See Sudden stratospheric warming, Polar vortexButler et al. 2017
Vortex split: A type of sudden stratospheric warming in which the polar vortex is stretched and pinched into two separate vortices, usually by a wave-2 pattern of planetary waves.Why it matters. Splits are often abrupt and affect the whole depth of the stratosphere at once. On our page a split is flagged when the vortex aspect ratio exceeds 2.4.See Vortex displacement, Sudden stratospheric warmingCharlton and Polvani 2007; Seviour et al. 2013
Vortex displacement: A type of sudden stratospheric warming in which the polar vortex is shoved off the pole, usually toward Eurasia or the North Atlantic, by a wave-1 pattern of planetary waves.Why it matters. Displacements are the commoner type. On our page a displacement is flagged when the vortex centre is south of 66°N.See Vortex split, Sudden stratospheric warmingCharlton and Polvani 2007; Seviour et al. 2013
Final warming: The spring breakdown of the polar vortex, after which the stratospheric winds stay easterly until the next autumn.Why it matters. It happens every year between March and May, so it is excluded from the list of sudden warmings; an early, abrupt final warming can still affect spring weather.See Sudden stratospheric warming, Polar vortexCharlton and Polvani 2007
Stratosphere–troposphere coupling: The way anomalies in the stratospheric polar vortex work their way down and change the jet stream, storm tracks and surface pressure patterns over the following weeks.Why it matters. It is the reason a weak or strong vortex matters for weather 2 to 8 weeks later, and a source of skill for week 3–6 forecasts; not every event couples down.See Sudden stratospheric warming, Northern annular mode, Arctic OscillationBaldwin and Dunkerton 2001; Kidston et al. 2015
Eddy heat fluxv′T′: The northward transport of heat by the waves in the flow, averaged around a latitude circle. At 100 hPa it measures how much planetary-wave activity is entering the stratosphere.Why it matters. A week or more of large positive values often comes before a weakening of the polar vortex.See Sudden stratospheric warming, Polar vortexPolvani and Waugh 2004
Geopotential height: The height above sea level of a given pressure surface (for example 500 hPa or 10 hPa), adjusted for the small variation of gravity. Upper-air charts draw it as contour lines.Why it matters. Low heights mark cold air and troughs, high heights warm air and ridges; winds blow roughly along the contours, faster where they are closer together.See Polar vortex, Jet streamWMO

Climate and the ocean

Niño-3.4 region: The box of the central-eastern equatorial Pacific from 5° N to 5° S and 170° W to 120° W. Its average sea-surface temperature anomaly is the main number used to track El Niño and La Niña. Neighbouring boxes are Niño 4 (west, 160° E–150° W), Niño 3 (east, 150°–90° W) and Niño 1+2 (off Peru and Ecuador, 0–10° S, 90°–80° W).Why it matters. Warming or cooling in this box is what best shifts tropical thunderstorms and, through them, the jet stream over North America.See Oceanic Niño Index, Relative Oceanic Niño Index, El Niño, La Niña, SST anomaly · On the site: ENSO trackerNOAA CPC; Trenberth 1997
Oceanic Niño IndexONI: The 3-month running mean of the Niño-3.4 sea-surface temperature anomaly (NOAA ERSST data), measured against 30-year base periods that move every five years. El Niño conditions: ONI at or above +0.5 °C; La Niña: at or below −0.5 °C. A historical El Niño or La Niña episode needs the threshold met for at least five overlapping 3-month seasons in a row.Why it matters. The ONI is how past events are dated and compared (1997–98, 2015–16 and so on). NOAA CPC now uses the relative version, RONI, for official monitoring.See Relative Oceanic Niño Index, Niño-3.4 region, El Niño, La Niña, Base period · On the site: ENSO trackerNOAA CPC
Relative Oceanic Niño IndexRONI: The Niño-3.4 anomaly minus the average anomaly of the whole tropical ocean (20° N–20° S), rescaled so it varies by the same amount as the ONI, then averaged over 3 months like the ONI. It measures whether the central Pacific is warm compared with the rest of the tropics, which is what drives the ENSO weather response.Why it matters. As the oceans warm everywhere, the traditional ONI drifts toward El Niño values even when the Pacific is not unusual compared with its surroundings. RONI removes that drift. The ENSO page uses it as the official index.See Oceanic Niño Index, Niño-3.4 region, El Niño, La Niña · On the site: ENSO trackervan Oldenborgh et al. 2021; NOAA CPC
Southern Oscillation IndexSOI: The standardized difference in sea-level pressure between Tahiti (central Pacific) and Darwin (northern Australia). Negative values (lower pressure at Tahiti, higher at Darwin) mean weaker trade winds and go with El Niño; positive values go with La Niña.Why it matters. The SOI is the atmosphere's half of ENSO. An ocean warming without a matching sustained negative SOI is a weakly coupled event that usually has less effect on weather far away.See El Niño, La Niña, Walker circulation, Trade winds · On the site: ENSO trackerNOAA CPC; Ropelewski and Jones 1987
SST anomaly: How much warmer (positive) or cooler (negative) the sea surface is than its long-term average for that place and time of year, in °C.Why it matters. Anomalies, not raw temperatures, show El Niño, La Niña and marine heatwaves: a 28 °C ocean is normal in the western Pacific but a strong El Niño signal in the eastern Pacific.See Sea-surface temperature, Base period, Niño-3.4 region · On the site: ENSO trackerGeneral usage
Base periodclimate normal: The run of years (usually 30) whose average is used as "normal" when an anomaly is calculated, for example 1991–2020.Why it matters. The same ocean can show a different anomaly against a different base period. NOAA's ONI moves its base period every five years so each value is measured against the climate of its own time.See SST anomaly, Climatology, Oceanic Niño IndexWMO; NOAA CPC
ENSO alert system: NOAA CPC's status labels for El Niño and La Niña. Watch: conditions are favourable for El Niño (or La Niña) to develop within about six months. Advisory: El Niño (or La Niña) conditions are observed and expected to continue. Final Advisory: the conditions have ended. Not active: neutral, with no event expected soon.Why it matters. The label in the status box at the top of the ENSO page is this official status, updated monthly in CPC's ENSO Diagnostic Discussion.See El Niño, La Niña, Oceanic Niño Index, Relative Oceanic Niño Index · On the site: ENSO trackerNOAA CPC
Oceanic Kelvin wave: A large, slow wave in the upper ocean that travels east along the equator, taking about two to three months to cross the Pacific. A downwelling Kelvin wave pushes the thermocline down and carries warm water east; an upwelling wave does the opposite.Why it matters. Westerly wind bursts in the western Pacific set off downwelling Kelvin waves, which often start or strengthen an El Niño; they show as eastward-sloping warm streaks on a Hovmöller diagram and as warm blobs moving east below the surface.See Thermocline, Hovmöller diagram, El Niño, La Niña · On the site: ENSO trackerGeneral usage
Thermocline: The layer in the ocean where temperature drops quickly with depth, separating warm surface water from cold deep water. Along the equatorial Pacific it is deep in the west (around 150 m or more) and shallow in the east (around 50 m), and is often tracked by the 20 °C isotherm.Why it matters. When the eastern thermocline deepens, upwelling brings up warmer water and the surface warms: El Niño. When it shoals, cold water reaches the surface more easily: La Niña. Heat stored below the surface leads the surface by months.See Oceanic Kelvin wave, El Niño, La Niña · On the site: ENSO trackerGeneral usage
Walker circulation: The east–west loop of air over the tropical Pacific: rising air and thunderstorms over the warm western Pacific and Indonesia, westward trade winds at the surface, eastward flow aloft and sinking air over the cool eastern Pacific.Why it matters. El Niño weakens or shifts the loop and La Niña strengthens it. Moving the tropical thunderstorms moves the heat source that steers the jet stream over the North Pacific and Canada.See Trade winds, Southern Oscillation Index, El Niño, La NiñaGeneral usage (after G. Walker)
Trade winds: The steady easterly winds of the tropics, blowing toward the equator from the subtropical highs. Over the Pacific they push warm surface water west and let cold water well up in the east.Why it matters. Weaker trades (and westerly wind bursts) let warm water slosh back east and start El Niño; stronger trades favour La Niña.See Walker circulation, Southern Oscillation Index, El Niño, La NiñaGeneral usage
Pacific Decadal OscillationPDO: A long-lived pattern of North Pacific sea-surface temperature. In its positive (warm) phase the water along the coast of North America is warmer than normal and the central North Pacific cooler; the negative phase is the reverse. Phases can last years to decades.Why it matters. A positive PDO tends to add to the mild-winter signal of El Niño in western Canada, and a negative PDO to the cold signal of La Niña; when they disagree, ENSO's usual effect is often weaker.See El Niño, La Niña, SST anomaly · On the site: ENSO trackerMantua et al. 1997
Pacific–North American patternPNA: A recurring pattern of the upper-level flow: a ridge over western North America with troughs over the North Pacific and the southeastern United States (positive phase), or the reverse (negative phase).Why it matters. El Niño tends to favour the positive PNA, which brings milder, drier winters to western Canada; La Niña tends to favour the negative phase, with colder air reaching BC and the Prairies more often.See El Niño, La Niña, Jet stream, Trough, ridgeWallace and Gutzler 1981
ENSO model plume: A chart with one line per forecast model showing its prediction of the Niño-3.4 anomaly for the coming overlapping 3-month seasons. IRI (Columbia University) collects the plume every month from about two dozen dynamical and statistical models.Why it matters. The middle of the plume shows where models think ENSO is heading; its width shows how much they disagree, which grows with lead time.See Dynamical model, Statistical model, Multi-model mean, Spring predictability barrier · On the site: ENSO trackerIRI
Spring predictability barrier: The drop in ENSO forecast skill for forecasts made in northern spring (about March to May). ENSO is weakest and most easily nudged by weather noise at that time of year, so forecasts made before or through spring are less reliable than those made in summer or autumn.Why it matters. A spring outlook for next winter's El Niño or La Niña deserves less trust than the same outlook issued in August.See ENSO model plume, Skill, Lead timeGeneral usage; Barnston et al. 2012
Anomaly: The difference between a value and its normal (the long-term average for that place, date and hour). A 500 hPa height anomaly of +15 dam means the 500 hPa surface is 150 m higher than normal: a ridge stronger than usual.See Climate normals, Climatology, SST anomaly, Base periodGeneral usage
Albedo: The share of sunlight a surface reflects: fresh snow 80–90 %, grass and forest 10–25 %, open water under 10 %. High albedo keeps snow-covered ground cold in sunshine, so fog and low cloud over snow burn off more slowly.See Snow cover, Burn-off, lift, Radiative coolingGeneral usage
Arctic OscillationAO: The leading pattern of surface pressure variability north of 20°N: a see-saw between the Arctic and the mid-latitudes. Negative AO means higher Arctic pressure and a weaker, wavier jet.Why it matters. Negative AO spells often bring cold outbreaks to southern Canada and the Great Lakes; positive AO keeps Arctic air bottled up in the north.See Northern annular mode, North Atlantic Oscillation, Teleconnection · On the site: Polar vortex and teleconnectionsThompson and Wallace 1998; NOAA CPC
Northern annular modeNAM: A ring-shaped pattern of pressure or height anomalies centred on the pole that appears at every level from the surface (where it is the AO) to the upper stratosphere.Why it matters. Following the NAM down from the stratosphere is how stratosphere–troposphere coupling is usually shown.See Arctic Oscillation, Stratosphere–troposphere couplingBaldwin and Dunkerton 2001
North Atlantic OscillationNAO: A pressure see-saw between the Icelandic low and the Azores high that controls the strength and path of the North Atlantic jet and storm track.Why it matters. Negative NAO, often with a block near Greenland, favours cold in Ontario, Quebec and the Maritimes and mild weather in Labrador and Baffin Island; positive NAO the reverse.See Arctic Oscillation, Blocking pattern, Teleconnection · On the site: Polar vortex and teleconnectionsHurrell 1995; NOAA CPC
Madden–Julian OscillationMJO: An eastward-moving pulse of tropical thunderstorms and winds that circles the globe in about 30 to 60 days, described by 8 phases on a phase diagram.Why it matters. Its tropical heating sends waves to the mid-latitudes, tilting Canadian winter temperatures about 10 to 20 days later, for example milder after phase 3 and colder after phase 7.See Teleconnection, Pacific–North American pattern · On the site: Polar vortex and teleconnectionsMadden and Julian 1971; Wheeler and Hendon 2004; Lin and Brunet 2009
Quasi-biennial oscillationQBO: The alternation of easterly and westerly winds in the equatorial stratosphere, with a period of about 28 months, descending slowly with time.Why it matters. When the QBO is easterly at 30–50 hPa the northern winter polar vortex tends to be weaker and sudden warmings more likely (the Holton–Tan effect).See Polar vortex, Sudden stratospheric warmingHolton and Tan 1980
Teleconnection: A recurring large-scale pattern linking weather in places thousands of kilometres apart, such as the AO, NAO, PNA or the influence of ENSO and the MJO.Why it matters. Teleconnections are a main source of skill for forecasts beyond about 10 days, giving tilts in the odds rather than day-by-day detail.See Arctic Oscillation, North Atlantic Oscillation, Pacific–North American pattern, Madden–Julian Oscillation, El Niño, La NiñaWallace and Gutzler 1981; NOAA CPC

Space weather

AuroraNorthern lights, aurora borealis: Light emitted 100–300 km up when electrons from Earth's magnetosphere hit oxygen (green and red light) and nitrogen (blue and purple). Over Canada it is most frequent under the auroral oval across the territories and the northern provinces, and it reaches farther south during geomagnetic storms.Why it matters. Strong geomagnetic storms that bring aurora south can also disturb HF radio and GNSS (GPS) on polar routes; NOAA and Space Weather Canada issue the warnings.See Auroral oval, Kp index, IMF Bz, OVATION aurora model · On the site: Aurora and night skyGeneral usage (NOAA SWPC, NRCan)
Auroral oval: The ring around each geomagnetic pole where aurora is most frequent. It widens and moves toward the equator as geomagnetic activity rises; in quiet times its southern edge lies over northern Canada.See Aurora, Geomagnetic latitude, View line, Kp index · On the site: Aurora and night skyGeneral usage (NOAA SWPC)
Kp indexPlanetary K index: A 0 to 9 measure of how disturbed Earth's magnetic field is, for each 3-hour period, from a network of magnetic observatories. Kp 5 or more is a geomagnetic storm (NOAA G1 at Kp 5, up to G5 at Kp 9).Why it matters. Roughly, each step up in Kp moves the auroral oval's southern edge about 2° of geomagnetic latitude farther south. Kp forecasts are often off by one or two units, especially for storm timing.See Geomagnetic storm scale, Aurora, Auroral oval · On the site: Aurora and night skyGeneral usage (NOAA SWPC, GFZ Potsdam)
Geomagnetic storm scaleNOAA G1–G5: NOAA's five-step storm scale tied to Kp: G1 minor (Kp 5), G2 moderate (Kp 6), G3 strong (Kp 7), G4 severe (Kp 8), G5 extreme (Kp 9). Higher levels bring aurora far south and can affect power grids, satellites, HF radio and GNSS.See Kp index, AuroraGeneral usage (NOAA SWPC space weather scales)
IMF BzNorth–south part of the interplanetary magnetic field: The solar wind carries a magnetic field (the IMF). Bz is its north–south component, in nanotesla (nT). When Bz points south (negative) it links with Earth's field and lets solar-wind energy in; a southward Bz of about −10 nT or more, lasting an hour or longer, is a strong sign of aurora.See Solar wind, Aurora, OVATION aurora modelGeneral usage (NOAA SWPC)
Solar wind: The stream of charged particles flowing out from the Sun, usually at 300–500 km/s and over 700 km/s in fast streams and after coronal mass ejections. Satellites at L1, about 1.5 million km sunward, measure it 30–60 minutes before it reaches Earth.See IMF Bz, Coronal mass ejection, OVATION aurora modelGeneral usage (NOAA SWPC)
Coronal mass ejectionCME: A huge cloud of magnetised solar plasma thrown out from the Sun. If it is aimed at Earth it arrives in roughly 1 to 4 days and can cause the strongest geomagnetic storms; its arrival time is typically uncertain by several hours either way.See Solar wind, Kp index, Geomagnetic storm scaleGeneral usage (NOAA SWPC)
OVATION aurora modelNOAA SWPC OVATION Prime: NOAA's model of where aurora is happening, driven by the live solar-wind measurements at L1. It gives the probability of aurora overhead about 30 to 90 minutes ahead: a nowcast, not a long-range forecast.Why it matters. Aurora is often visible low on the northern horizon several hundred kilometres south of where OVATION shows it overhead, which is why the aurora page adds view-line geometry.See View line, IMF Bz, Aurora · On the site: Aurora and night skyGeneral usage (NOAA SWPC)
View line: The southernmost place from which aurora may be seen low on the northern horizon, south of the area where it is overhead. Aurora 100–250 km up can stay above the horizon for observers several degrees of latitude farther south.See Auroral oval, OVATION aurora modelGeneral usage (NOAA SWPC)
Geomagnetic latitude: Latitude measured from the geomagnetic pole (in the Canadian Arctic, near 81° N 73° W) instead of the geographic pole. Aurora follows geomagnetic latitude, so Canada sits closer to the auroral oval than Europe at the same geographic latitude.See Auroral oval, AuroraGeneral usage (NRCan, NOAA)

Wildfire and smoke

Fire Weather Index SystemFWI System, part of the CFFDRS: Canada's daily fire-weather rating system (Van Wagner 1987). From noon temperature, humidity, wind and 24-hour rain it keeps three fuel-moisture codes (FFMC, DMC, DC) and three fire-behaviour indices (ISI, BUI, FWI).Why it matters. Agencies across Canada and many countries use it to set preparedness, fire bans and staffing. The codes carry memory: a week of dry weather builds up, a big rain resets the fast codes but not the slow ones.See Fine Fuel Moisture Code, Duff Moisture Code, Drought Code, Initial Spread Index, Buildup Index, Fire Weather Index · On the site: Wildfire and smokeVan Wagner (1987), Canadian Forest Service
Fire Weather IndexFWI: One number for the potential intensity of a spreading fire, combining the spread index (ISI) and the fuel available (BUI). Generic classes: under 5 low, 5–10 moderate, 10–20 high, 20–30 very high, 30+ extreme.See Initial Spread Index, Buildup Index, Fire danger class · On the site: Wildfire and smokeVan Wagner (1987)
Fine Fuel Moisture CodeFFMC: How dry the surface litter and fine fuels are (0–101). It reacts within a day. Above about 86 fires start easily; above 92 almost any spark catches.See Fire Weather Index System, Initial Spread IndexVan Wagner (1987)
Duff Moisture CodeDMC: Moisture of the loose decomposing organic layer a few centimetres down (about a 12-day time lag). High values let lightning strikes smoulder for days (holdover fires).See Fire Weather Index System, Holdover fire, Buildup IndexVan Wagner (1987)
Drought CodeDC: Deep drying of compact organic soil and large logs (about a 52-day time lag). Over 300 means deep, long-burning fires; it needs heavy rain to come down.See Overwintered Drought Code, Buildup IndexVan Wagner (1987)
Initial Spread IndexISI: The expected rate of fire spread from wind and fine-fuel dryness. It roughly doubles for every 14 km/h of extra wind.See Fine Fuel Moisture Code, Fire Weather Index, Rate of spread (fire)Van Wagner (1987)
Buildup IndexBUI: The total fuel available for burning, combined from the DMC and DC. Over 60 deep fuels add a lot of energy to a fire.See Duff Moisture Code, Drought Code, Fire Weather IndexVan Wagner (1987)
Fire danger class: The FWI grouped into low, moderate, high, very high and extreme. National maps use generic breakpoints; provinces tune their own by region and fuel.See Fire Weather IndexCWFIS
Fire Behaviour Prediction SystemFBP System: The Canadian system that turns the FWI codes, wind, slope and a fuel type into rate of spread, fuel consumed, head fire intensity and crown fraction burned.See FBP fuel type, Head fire intensity, Rate of spread (fire), Crown fraction burnedForestry Canada Fire Danger Group (1992); Wotton et al. (2009)
FBP fuel type: One of 16 standard Canadian fuel complexes, e.g. C-2 boreal spruce, C-3 mature jack or lodgepole pine, D-1 leafless aspen, M-1/M-2 boreal mixedwood, O-1 grass. Each burns with its own spread and consumption equations.See Fire Behaviour Prediction SystemForestry Canada (1992)
Head fire intensityHFI, kW/m: Energy released per metre of the fire's leading edge (Byram intensity). Under 2000 kW/m crews can attack directly; over 4000 it is beyond direct attack; over 10 000 continuous crown fire with long-range spotting.See Fire Behaviour Prediction System, Crown fraction burnedByram (1959); CIFFC intensity classes
Rate of spread (fire)ROS, m/min: How fast the head of a fire moves. 1 m/min is a slow surface fire; 30–50 m/min (2–3 km/h) is a running crown fire in boreal spruce.See Initial Spread Index, Fire Behaviour Prediction System, Length-to-breadth ratioForestry Canada (1992)
Crown fraction burnedCFB: The share of tree crowns a fire is expected to burn: 0 surface fire, 0.1–0.9 intermittent crowning (torching), above 0.9 continuous crown fire.See Head fire intensity, Fire Behaviour Prediction SystemVan Wagner (1977); Forestry Canada (1992)
Length-to-breadth ratioL:B (fire): How elongated a wind-driven fire becomes. Calm fires grow roughly round (L:B near 1); in strong wind they become long, narrow ellipses (L:B of 3–6).See Rate of spread (fire)Forestry Canada (1992)
Continuous Haines indexC-Haines: A 0–13 measure of how much dry, unstable air in the lower mid-levels (850–700 hPa) helps a fire's smoke plume grow. 10 or more supports plume-dominated, erratic fire behaviour.See Pyrocumulus, pyroCbMills & McCaw (2010)
PM2.5Fine particulate matter: Particles 2.5 µm across or smaller, measured in µg/m³. Wildfire smoke is mostly PM2.5; it reaches deep into the lungs. Above about 35 µg/m³ is unhealthy for sensitive people, above 250 hazardous.See AQHI+, Wildfire smoke, FireWorkECCC; US EPA AQI breakpoints
AQHI+Air Quality Health Index plus: A PM2.5-only version of the AQHI used in wildfire smoke: the 1-hour PM2.5 in µg/m³ divided by 10, rounded up. 1–3 low risk, 4–6 moderate, 7–10 high, 11 very high.See PM2.5ECCC / provincial health agencies
FireWorkECCC smoke forecast: Environment and Climate Change Canada's wildfire smoke forecast: fire emissions estimated from satellite hotspots, carried by the RAQDPS air-quality model (10 km), hourly to 72 hours.See PM2.5, Wildfire smoke · On the site: Wildfire and smokeECCC
Hotspot (satellite): A satellite pixel much warmer than its surroundings in the 4 µm band, flagged as a probable fire (VIIRS 375 m, MODIS 1 km). It means a fire somewhere in the pixel, not a fire that size; gas flares and industry also show up.See M3 fire perimeterNASA FIRMS; CWFIS
M3 fire perimeter: CWFIS's estimated fire outline drawn around clusters of satellite hotspots through the season. Useful for remote fires; it can include unburned islands and lag a fast-moving front.See Hotspot (satellite), CWFISCWFIS
Stage of control: The agency's status for a fire: out of control (still spreading), being held (not expected to spread past control lines with current resources), under control (no further spread), or out. Remote fires may be on a monitored or modified response.See CIFFCCIFFC
CWFISCanadian Wildland Fire Information System: Natural Resources Canada's national fire information system: daily fire-weather maps, hotspots, perimeters, fuel maps and the national fire database.See Canadian National Fire Database, M3 fire perimeterNatural Resources Canada
CIFFCCanadian Interagency Forest Fire Centre: The national centre that coordinates fire-fighting resources between provinces, territories, Parks Canada and other countries, and compiles the national fire situation.See Stage of controlCIFFC
Canadian National Fire DatabaseCNFDB / NFDB: Agency fire records for Canada compiled by the Canadian Forest Service: location, date, cause and size of each fire, back to the 1940s for some agencies (most complete since about 1980).See CWFIS, Fire occurrence predictionCanadian Forest Service
Overwintered Drought Code: The spring start-up Drought Code, carried over from the fall value and reduced by winter precipitation (Lawson & Armitage 2008). A dry autumn with little snow can start the next fire season already in deep drought.See Drought Code · On the site: Wildfire and smokeLawson & Armitage (2008)
Fire occurrence predictionFOP: A statistical forecast of how many new fires will start, or the chance of a start in an area, from fire weather, lightning, season and human activity. Agencies use it to place crews and aircraft before fires are reported.See Canadian National Fire Database, ROC curve, Reliability · On the site: Wildfire and smokeWoolford et al. (2021); Wotton et al. (2003)
Holdover fireSleeper fire: A lightning-started fire that smoulders in duff for days (or over a winter, as an overwintering fire) before it flares up and is detected. More likely when the DMC and DC are high.See Duff Moisture Code, Overwintered Drought CodeGeneral usage (CFS)

Weather models

Run, init: One start of a model. The 18Z run started from the state of the atmosphere at 1800 UTC and takes hours to arrive, so a run's age is counted from that time. F012 means 12 hours after the start.This site
RDPS: Regional Deterministic Prediction System, the Canadian government's regional model. About 10 km grid here.General usage · grid size from this site
HRDPS: High Resolution Deterministic Prediction System, the Canadian high-resolution model. About 2.5 km grid.General usage · grid size from this site
RRFS: Rapid Refresh Forecast System, a US NOAA model. About 13 km grid here.General usage · grid size from this site
RAP: Rapid Refresh, a US NOAA model updated every hour. About 32 km grid here.General usage · grid size from this site
ECMWF: European Centre for Medium-Range Weather Forecasts. We use its open data, about 25 km grid.General usage · grid size from this site
UKMO: The UK Met Office global model. About 10 km grid.General usage · grid size from this site
BLEND: Our combined forecast from RDPS, HRDPS, RRFS, RAP, ECMWF and UKMO. It's rebuilt when a member has something newer, and at least hourly.This site
FRONTIER: Our most detailed experimental forecast, for selected western airports. It starts from a blend weighted by each model's recent skill, then uses recent observations, satellite, radar, lightning, pilot reports and soundings, and it learns from how past forecasts verified. It's guidance, not an issued forecast.This site
METAR-adjusted: A model's airport forecast corrected with the latest METAR, so the first hours start from what's actually happening.This site
Guidance: Computed output from MeteoFrontier. It isn't an issued NAV CANADA or ECCC product.This site
Deterministic model: A single model run with one answer, as opposed to an ensemble of many runs that shows the spread of possible outcomes. All our maps are deterministic.General usage
Ensemble: Many runs of a model with slightly different starting points. When the members agree, confidence is higher.General usage
Grid spacingResolution: The distance between model grid points. A 2.5 km model can show valleys and lake effects that a 25 km model smooths out, but finer is not always more accurate.General usage
Lead timeForecast hour: How far ahead of the run start a forecast is valid. Skill falls as lead time grows.General usage
Valid time: The moment a forecast map describes. Run time plus forecast hour.General usage
Bias correction: Adjusting a model's output for errors it makes over and over, such as always being too warm at one airport.General usage
Climatology: What usually happens at a place and time of year. A forecast that just says the usual is a baseline other forecasts are measured against.General usage
Analogs: Past situations that looked like today. What happened then is a hint for what could happen now.General usage
Calibrated: A probability that has been checked against what happened, so 30 % really does verify about 30 % of the time. Uncalibrated indices on this site say so in their legend.General usage · This site
Model climatology rarity: How unusual a model value is compared with that model's own past values at the same level. Our turbulence maps colour by rarity, not by a measured intensity.This site
AI, learned: Output from a machine-learning model trained on past forecasts and observations. Labelled unverified until it has been scored live against METARs.This site
NWPNumerical weather prediction: Forecasting by computer: the atmosphere is divided into a 3-D grid, today's state is estimated at every point, and the equations of motion, heat and moisture are stepped forward in time.See Deterministic model, Ensemble, Data assimilation · Learn: How weather models differGeneral usage
Data assimilation: Blending the latest observations (balloons, aircraft, satellites, surface stations, radar) with a short previous forecast to make the best estimate of the atmosphere right now, the starting point for the next run.Why it matters. Where observations are sparse, as over the Pacific and the North, the starting point is less certain and so is the forecast.See Analysis, NWP, Run, init · Learn: How weather models differGeneral usage
Analysis: A model's best picture of the current atmosphere, made by data assimilation. Also used for a hand- or machine-drawn surface map of fronts and pressure.See Data assimilation, Surface analysisGeneral usage
Spin-up: The first few hours of a model run, when clouds, precipitation and small-scale circulations are still settling in after the start.Why it matters. The first hour or two of precipitation and cloud from a fresh run can be less reliable than hours a little later.See Run, init, Lead timeGeneral usage
Parameterisation: A simplified recipe for processes too small or complex for the model grid to resolve directly, such as turbulence, cloud microphysics, radiation and, in coarser models, convection.Why it matters. Many model errors in fog, low cloud and drizzle come from these recipes rather than from the large-scale flow.See Grid spacing, Microphysics scheme, Boundary-layer scheme · Learn: How weather models differGeneral usage
Convection-allowing model: A model with a grid of about 3 km or finer, fine enough to draw individual thunderstorms instead of estimating their effect. HRDPS, HRRR and RRFS are examples.Why it matters. They show storm modes and timing better, but an individual storm in the model is a possibility, not a promise of a storm in that exact spot.See HRDPS, HRRR, RRFS, Grid spacing · Learn: How weather models differGeneral usage
Microphysics scheme: The part of a model that handles cloud droplets, ice, snow, rain, graupel and how they grow, melt and fall.See Parameterisation, Supercooled liquid waterGeneral usage
Boundary-layer scheme: The part of a model that handles mixing near the ground: how heat, moisture and momentum spread through the lowest few thousand feet.Why it matters. It decides how well the model gets night inversions, fog, gusts and the afternoon mixing depth.See Parameterisation, PBL, InversionGeneral usage
Model terrain: The ground surface as the model sees it, smoothed to its grid. Valleys come out shallower and peaks lower than they really are.Why it matters. In the mountains, a model's surface temperature, wind and cloud base can be off because its 'valley floor' may be hundreds of metres above the real one.See Grid spacing, Channelling · Learn: How weather models differGeneral usage
Vertical levels: The layers a model is divided into in the vertical, usually packed closer together near the ground. Output is often given only on a few standard pressure levels.Why it matters. A thin inversion or a shallow freezing-drizzle cloud can fall between output levels and be missed on charts that use only four or five levels.See 850, 700, 500, 250 hPa, InversionGeneral usage
Grid point, grid box: The points where a model keeps its values. Each value stands for an average over a box around the point, not for one spot.See Grid spacing, Representativeness, InterpolationGeneral usage
Interpolation: Estimating a value between grid points or between output times from the ones on either side.See Grid point, grid boxGeneral usage
Forecast hour: Hours since the run started, written as F00, F06, +12 and so on. Valid time = run time + forecast hour.See Run, init, Valid time, Lead time · On the site: ModelMapsGeneral usage
Update cycle: How often a model runs. RAP, HRRR and RRFS run every hour; HRDPS and RDPS every 6 hours; global models every 6 or 12 hours.Why it matters. An hourly model has seen more recent observations but isn't automatically more accurate at longer lead times.See Run, init, RAP, HRDPS · Learn: How weather models differGeneral usage
GDPSGlobal Deterministic Prediction System: ECCC's global model, run twice a day, used for the medium range and to feed the regional models at their edges.See RDPS, HRDPS, GEPSGeneral usage
GEPSGlobal Ensemble Prediction System: ECCC's global ensemble: many runs of a global model, each started slightly differently, to show the range of possible outcomes.See Ensemble, GDPS, Ensemble spreadGeneral usage
GFSGlobal Forecast System: The US National Weather Service global model, run four times a day.See GDPS, ECMWFGeneral usage
HRRRHigh-Resolution Rapid Refresh: NOAA's hourly 3 km convection-allowing model over the US and southern Canada, nested in RAP. RRFS is its successor.See RAP, RRFS, Convection-allowing modelGeneral usage
NAMNorth American Mesoscale model: An older NOAA regional model for North America, run four times a day. RRFS is its successor.See RRFS, RAPGeneral usage
AI weather models: Forecast models trained on decades of past analyses that predict the next state of the atmosphere directly, without solving the physics equations step by step. ECMWF's AIFS is one now run operationally.Why it matters. They're fast and often skilful for large-scale patterns, but still have to prove themselves on small-scale aviation hazards such as fog and icing.See ECMWF, AI, learned, NWPGeneral usage
MOSModel output statistics: Statistical equations, built from years of past model forecasts and matching observations, that turn raw model output into forecasts for specific sites, fixing systematic errors.See Bias correction, Post-processingGeneral usage
Post-processing: Everything done to raw model output after the run: bias correction, statistical models, blending several models, or machine learning.Why it matters. Most of the forecasts on this site are post-processed: raw models plus corrections learned from how they have verified.See MOS, Bias correction, BLENDGeneral usage
Downscaling: Getting local detail from a coarse forecast, either with a finer model nested inside it or with statistics that relate the large scale to local weather.See Post-processing, Grid spacingGeneral usage
Nowcasting: Forecasting the next few hours mainly from the latest observations, radar, satellite and lightning, by tracking and extrapolating what's already there.Why it matters. For the first hour or two, extrapolating a fog edge or a line of storms often beats a model.See Persistence, ExtrapolationGeneral usage
Extrapolation: Assuming that what's moving keeps moving the same way at the same speed. Works well for short times, then fails as things grow, decay or turn.See Nowcasting, PersistenceGeneral usage
Ensemble spread: How far apart the members of an ensemble are. Small spread means the outcome is more predictable; large spread means the forecast is uncertain.See Ensemble, Ensemble members, Forecast uncertainty · Learn: Probability and verification: what 30 % meansGeneral usage
Ensemble members: The individual runs of an ensemble. One, the control, starts from the best analysis; the others start from slightly changed states or use varied model settings.See Ensemble, Ensemble spreadGeneral usage
Time-lagged ensemble: Treating several recent runs of the same model, valid at the same time, as a small ensemble.Why it matters. If the last four hourly runs keep jumping around, confidence should be low; if they agree, a little higher.See Ensemble, Ensemble spread, Run, initGeneral usage
Predictability: How far ahead something can be usefully forecast. Large weather systems are predictable for days; a single thunderstorm or fog patch only for hours, because small errors grow fast.See Lead time, Ensemble spread · Learn: How weather models differGeneral usage
ReforecastHindcast: Re-running a model, or a forecast method, over past dates to see how it would have done and to train statistical corrections.See MOS, VerificationGeneral usage
Model consensus: Where several independent models agree. Agreement raises confidence; disagreement is useful information in itself.See BLEND, EnsembleGeneral usage
Run-to-run consistency: How much a model's forecast for the same valid time changes from one run to the next. Steady runs suggest a predictable situation; runs that flip suggest the model is on a knife edge and the forecast should carry that uncertainty.See Time-lagged ensemble, Ensemble spread, Predictability · Learn: Using NWP and AI guidance operationallyGeneral usage
Overriding guidance: A forecaster's decision to depart from model or AI guidance, normally because current observations already contradict it, a known local effect is missing from it, or the guidance has a documented bias for this situation and season.Why it matters. Done well, it adds skill in the first hours and in terrain; done on a hunch, it usually subtracts it.See Guidance, Frequency bias, Persistence, Nowcasting · Learn: Using NWP and AI guidance operationallyGeneral usage
Data drift: A slow change in the data a statistical or AI model receives compared with the data it was trained on: a sensor replaced, a model upgraded, a new runway, a changed climate. The model's skill decays without any visible error.Why it matters. It is why AI guidance is re-verified continuously and retrained, and why a retrained model is first run in shadow next to the old one.See AI weather models, Bias correction, Verification · Learn: Using NWP and AI guidance operationallyGeneral usage
Dynamical modelclimate forecast: A forecast model that solves the physical equations of the ocean and atmosphere on a computer, like a weather model run for months ahead, usually as a coupled ocean–atmosphere system. Examples include NOAA CFSv2, ECMWF SEAS5 and ECCC's CanSIPS.Why it matters. Dynamical models handle situations without a close match in the historical record and have generally been the more skilful ENSO forecasters in recent decades, though not always.See Statistical model, ENSO model plume, NWPGeneral usage; Barnston et al. 2012
Statistical modelclimate forecast: A forecast method built from relationships found in past data, for example regression on current ocean temperatures and winds, analogue years or machine learning, rather than from the physical equations.Why it matters. Cheap and often competitive at short leads, but it can miss events unlike those in its training years.See Dynamical model, ENSO model plumeGeneral usage
Multi-model mean: The average of the forecasts from several different models. Errors that differ from model to model partly cancel, so the mean is usually more accurate than a typical single model.Why it matters. The mean hides the spread: a plume can have a moderate mean while half the models are strong and half weak. Look at both.See Ensemble, Ensemble spread, ENSO model plumeGeneral usage
REPSRegional Ensemble Prediction System: Environment and Climate Change Canada's regional ensemble: about 20 slightly different runs (members) of a 10 km model over North America, run twice a day, used to show the range of possible outcomes.Why it matters. Where REPS members spread widely (for example 2 to 25 cm of snow), the forecast is uncertain even if one deterministic model looks confident.See Ensemble, Ensemble members, GEFS, Ensemble spread · On the site: Winter Storm LabECCC
GEFSGlobal Ensemble Forecast System: NOAA's global ensemble: about 30 members of the US global model (GFS) started from slightly different conditions, at about 0.25–0.5° resolution, run four times a day out to 16 days.Why it matters. Coarser than regional ensembles, so it smooths mountains and lake-effect bands, but it gives an independent second opinion on the spread of outcomes.See Ensemble, Ensemble members, REPS, Ensemble spread · On the site: Winter Storm LabNOAA NCEP
Cloud fraction: The share of a model grid box (or a layer of it) that the model considers cloudy, from 0 to 100 %. Many products derive it from relative humidity; it is not the same as the okta coverage an observer reports.See Okta, RH, Cloud baseGeneral usage
Reanalysis: A consistent reconstruction of past weather made by running one fixed modern forecast system over decades of archived observations, giving complete gridded fields for every day.Why it matters. Climatologies, past-event catalogues and composites (such as our sudden-warming history) are built from reanalyses like NCEP/NCAR R1, ERA5 or MERRA-2; different reanalyses disagree slightly.See Climatology, AnomalyKalnay et al. 1996; Gelaro et al. 2017

Satellite

Infrared (IR): Satellite images of heat given off by cloud tops and the ground. Colder (higher) tops look brighter or more colourful. They work day and night.General usage
Visible: Satellite images of reflected sunlight, like a black-and-white photo. They show fine detail but only in daylight.General usage
Water vapour imagery: Images of moisture in the middle and upper atmosphere, not cloud. Dark areas are dry air aloft, often sinking air behind a system; bright areas are moist.General usage
Brightness temperature: The temperature a satellite sees at one wavelength. For a thick cloud it is close to the cloud-top temperature; thin cloud lets warmer heat from below through.General usage
ABIAdvanced Baseline Imager: The 16-channel imager on GOES-18 and GOES-19 behind all our satellite products.General usage
Channel, band: One wavelength the satellite measures, named like C13 (10.3 µm infrared). Each channel shows something different.General usage
L2 product: A NOAA retrieval made from the raw channels, such as cloud-top height or phase. It is computed, so it can be wrong in tricky scenes such as multi-layer cloud.General usage
Cloud-top phase: Whether the top of a cloud is liquid water, supercooled liquid, ice or a mix, retrieved from several channels. It is only the top: the cloud below can differ.General usage
Parallax: Tall clouds seen at an angle appear displaced away from the satellite, a few kilometres to tens of kilometres at Canadian latitudes. High tops look further north than they are.General usage
Fog difference10.3 − 3.9 µm: At night, water-droplet fog and stratus look colder at 3.9 µm than at 10.3 µm, so the difference picks them out. It does not work in daylight, when 3.9 µm also sees reflected sunlight.General usage
Cloud-top cooling: How fast cloud tops get colder between scans. Rapid cooling means a cloud is growing upward, often before it becomes a thunderstorm.General usage
Scan: One image from the satellite. The timeline on the satellite page shows the minutes between scans, so a gap in the loop is easy to spot.This site
Day–night boundary: Where daylight products stop working and night products take over. Combined day/night products can change look across it.General usage
Weather radar: A ground radar that sees precipitation, not cloud. Our radar products are ECCC's radar composite drawn over the infrared image.General usage · This site
Geostationary satellite: A satellite about 35,800 km above the equator that circles the Earth once a day, so it stays over the same spot and can image the same area every few minutes.Why it matters. Rapid, continuous pictures make it ideal for watching fog, storms and cloud move, but it views Canada at a slant from far to the south.See GOES, Polar-orbiting satellite, Viewing angle · Learn: Satellite channels, RGBs and radarGeneral usage
Polar-orbiting satellite: A satellite in a low orbit, around 800 km up, passing near both poles. It sees a strip of the Earth at a time, with finer detail, and covers high latitudes many times a day.Why it matters. In the Arctic, where geostationary views are stretched and slanted, polar passes (such as VIIRS) give the clearest pictures, but only a few times a day.See Geostationary satellite, Viewing angleGeneral usage
GOES-East, GOES-West: The two US geostationary satellites covering the Americas: GOES-19 as GOES-East near 75° W and GOES-18 as GOES-West near 137° W. West has the better view of BC and Yukon, East of eastern Canada.See GOES, Geostationary satellite, ABI · On the site: Satellite · Learn: Satellite channels, RGBs and radarGeneral usage
Viewing angle: How slanted the satellite's view is. Over northern Canada a geostationary satellite looks at a low angle, so pixels are stretched and tall clouds appear displaced away from the satellite.Why it matters. Fog and cloud edges in the North are less precise, and cloud tops look shifted north of where the cloud really is.See Parallax, Geostationary satellite · Learn: Satellite channels, RGBs and radarGeneral usage
Pixel size: The smallest area a satellite image can resolve. For GOES ABI it's about 0.5 km for the red visible channel, 1 km for some others and 2 km for infrared, at the point right below the satellite, and larger toward the edges.See ABI, Viewing angle, Channel, bandGeneral usage
Shortwave infrared, 3.9 µm: An infrared channel that sees both heat given off and, by day, sunlight reflected. At night, low water clouds look colder in it than in longwave IR, which is the basis of the night fog difference.Why it matters. It also picks out hot spots such as wildfires and, by day, small-droplet cloud tops.See Fog difference, Infrared (IR), Night fog difference · On the site: Satellite: Shortwave infrared (3.9 µm) · Learn: Satellite channels, RGBs and radarGeneral usage
Night fog difference: The 10.3 µm minus 3.9 µm temperature difference at night. Water clouds made of small droplets, such as fog and stratus, stand out because they emit less at 3.9 µm.Why it matters. It's the main night-time tool for spotting fog and low stratus, but it can't see fog under higher cloud, and it fades out near dawn.See Fog difference, Shortwave infrared, 3.9 µm, Multi-layer cloud · On the site: Satellite: Night fog difference · Learn: Satellite channels, RGBs and radarGeneral usage
Split window: The difference between two longwave infrared channels near 10.3 and 12.3 µm. Moisture, thin cirrus, dust and volcanic ash each change it in their own way.See Infrared (IR), VA, Ash RGBGeneral usage
Cirrus channel, 1.38 µm: A daytime channel that water vapour in the lower atmosphere blocks, so only high, thin ice cloud shows up.See CI, CS, CC, Channel, band · On the site: Satellite: Cirrus channel (1.38 µm)General usage
Snow/ice channel, 1.61 µm: A daytime channel where ice and snow absorb and look dark, while water cloud reflects and looks bright.Why it matters. It separates fog and liquid cloud from snow on the ground and from ice cloud, which look alike in visible imagery.See Visible, Cloud-top phase, Day snow–fog RGB · On the site: Satellite: Snow / ice channel (1.61 µm)General usage
Reflectance: The share of sunlight a surface or cloud sends back to the satellite. Thick cloud and fresh snow reflect a lot; water and forest little.See Visible, Snow/ice channel, 1.61 µmGeneral usage
Sun glint: Bright reflection of the sun off water, seen in visible and near-infrared images where the sun, the water and the satellite line up.See VisibleGeneral usage
Colour enhancement: A colour table applied to infrared images so cold cloud tops stand out in bright colours while warmer surfaces stay grey.See Infrared (IR), Brightness temperature, Cloud top · On the site: Satellite: Enhanced infraredGeneral usage
GLMGeostationary Lightning Mapper: An instrument on GOES that detects lightning flashes from space, including in-cloud lightning, by spotting brief flashes of light at cloud top.See Flash, stroke, CLDN, GOESGeneral usage
Full disk, CONUS, mesoscale: GOES scan areas: the whole visible Earth every 10 minutes, a large North American sector every 5 minutes, and two small movable sectors every minute.See Scan, GOESGeneral usage
Air mass RGB: A colour product from water vapour and infrared channels that shows upper-level air masses: warm, moist tropical air greenish, cold polar air bluish-purple, and dry, sinking air with high ozone (often near jet streaks and tropopause folds) reddish.See RGB product, Tropopause fold, Dry slot · On the site: Satellite: Air mass RGB · Learn: Satellite channels, RGBs and radarGeneral usage
Nighttime microphysics RGB: A night colour product mixing three infrared channels. Low water cloud and fog show in aqua or grey-green tones, high ice cloud in reds and darker colours, and clear ground in shades depending on temperature.See RGB product, Night fog difference · On the site: Satellite: Nighttime microphysics RGB · Learn: Satellite channels, RGBs and radarGeneral usage
Day snow–fog RGB: A daytime colour product that separates snow on the ground (red-orange) from fog and low water cloud (pale yellow to white).See RGB product, Snow/ice channel, 1.61 µm · On the site: Satellite: Day snow–fog RGB · Learn: Satellite channels, RGBs and radarGeneral usage
Ash RGB: A colour product built to pick out volcanic ash and sulphur dioxide plumes from ordinary cloud.See VA, Split window · On the site: Satellite: Ash RGBGeneral usage

Radar

Doppler radar: A radar that also measures how fast targets move toward or away from it (radial velocity), from the shift in frequency of the returned signal.Why it matters. Velocity shows wind shifts, rotation in storms and outflow boundaries that reflectivity alone can't.See Weather radar, Radial velocity, Velocity coupletGeneral usage
Radial velocity: The part of the target's motion directly toward or away from the radar. Wind blowing across the beam shows as zero, even if strong.See Doppler radar, Velocity coupletGeneral usage
Velocity couplet: Strong inbound and outbound velocities side by side in a storm, the radar sign of rotation such as a mesocyclone or tornado.See Radial velocity, Supercell, Hook echoGeneral usage
Dual polarisation: Radar that sends pulses with both horizontal and vertical orientation. Comparing the returns tells it about the shape and variety of the targets: big flattened raindrops, tumbling hail, wet snow, birds or insects. Canada's network was replaced with dual-polarisation S-band radars in recent years.See ZDR, CC, Weather radarGeneral usage
ZDRDifferential reflectivity: The ratio of horizontal to vertical return. Large raindrops are flattened and give high ZDR; hail tumbles and gives values near zero; dry snow is low.See Dual polarisation, CCGeneral usage
CCCorrelation coefficient: How alike the horizontal and vertical returns are from pulse to pulse. Uniform rain or snow gives values near 1; mixed targets such as melting snow, hail or debris give lower values.Why it matters. A ring of low CC marks the melting layer, and a small blob of very low CC in a rotating storm can be tornado debris.See Dual polarisation, Bright bandGeneral usage
Bright band: A ring or layer of high reflectivity at the melting level, where snowflakes get a coat of water and look to the radar like huge raindrops.Why it matters. It shows the height of the melting layer, and it can make rain rates there look too high.See Melting layer, Reflectivity, CCGeneral usage
Beam height: Radar beams go out in nearly straight lines while the Earth curves away, so the beam is higher above the ground the further it goes: thousands of feet up at 100 km.Why it matters. Shallow snow and drizzle far from the radar can pass underneath the beam and not show at all.See Beam overshooting, Radar coverage gaps · Learn: Satellite channels, RGBs and radarGeneral usage
Beam overshooting: When the radar beam passes over the top of shallow precipitation, so the radar shows nothing or too little even though it's snowing or drizzling.Why it matters. Lake-effect snow, upslope drizzle and freezing drizzle are often shallow, so radar badly underestimates them at range.See Beam height, FZDZ environmentGeneral usage
Beam blockage: Mountains or buildings blocking part of the beam, leaving wedge-shaped gaps or weak areas behind them.Why it matters. In BC's mountains, large areas are partly hidden from radar.See Beam height, Radar coverage gapsGeneral usage
Radar coverage gaps: Areas with no useful radar coverage, either beyond range, behind terrain or simply without a radar: most of northern Canada and much of the BC interior at low levels.See Beam blockage, Beam heightGeneral usage
Ground clutter: Echoes from the ground, buildings, wind turbines or the sea rather than precipitation. Processing removes most of it, but not all.See Anomalous propagationGeneral usage
Anomalous propagation: When a strong low-level inversion bends the radar beam down toward the ground, making false echoes that look like precipitation, often on calm, clear nights.See Ground clutter, InversionGeneral usage
Echo tops: The highest altitude where the radar detects a set reflectivity. Higher tops mean stronger, deeper storms.See Reflectivity, Cloud topGeneral usage
VILVertically integrated liquid: An estimate of the total water in a column from radar reflectivity at all heights. High values often mean heavy rain or hail.See Reflectivity, GR, GSGeneral usage
CAPPI, PPI: A PPI is one sweep at one beam angle, so its height grows with distance. A CAPPI combines sweeps to show reflectivity at a constant height above the ground, such as 1.5 km.See Composite reflectivity, Beam heightGeneral usage
Rain rate from radar: Radar measures reflectivity, not rainfall. A rule linking the two (a Z–R relation) converts it, which works best in steady rain and worst with hail, bright band or snow.See Reflectivity, Bright band · On the site: Satellite: Radar rain rateGeneral usage
Snow on radar: Dry snow reflects much less than rain, so heavy snow can look weak on radar. Wet snow near the melting level can look stronger than it is.See Bright band, Beam overshooting · On the site: Satellite: Radar snow rateGeneral usage
Attenuation: Weakening of the radar beam as it passes through heavy rain or hail, so storms behind a strong storm look weaker than they are.See ReflectivityGeneral usage

Maps, satellite and soundings

Theta-wθw, wet-bulb potential temperature: A way of labelling an air mass that barely changes as air rises, sinks, condenses or evaporates. Tight bands of change on a map often mark fronts.General usage
GOES: The US weather satellites behind our satellite page: GOES-18 and GOES-19.This site
RGB product: A satellite image that mixes several channels into colours to bring out things like cloud type or fog. Each has its own colour key. Ours are screening products, not observations.This site
Sounding: A vertical profile of temperature, moisture and wind. Observed soundings come from weather balloons (radiosondes), normally launched at 00Z and 12Z.This site
Tephigram: A chart of a sounding's temperature and dew point against height.General usage
Hodograph: A chart showing how wind speed and direction change with height.General usage
T − Td: Temperature minus dew point. The smaller the gap, the closer the air is to saturation. We mark moist layers at 3 °C or less, a first look at cloud and icing layers, not observed cloud.This site
Skew-T: A sounding diagram with temperature lines slanted to the right, so the atmosphere's profile stands more upright and details are easier to see. Widely used in North America; the tephigram is similar.See Tephigram, Sounding, Lapse rate · On the site: Observed soundings · Learn: Reading a soundingGeneral usage
Wind barb: The symbol for wind on charts: a staff pointing into the direction the wind comes from, with a half line for 5 KT, a full line for 10 KT and a pennant for 50 KT.See Wind direction, KTGeneral usage
Station plot: The cluster of numbers and symbols at each station on a surface map: temperature, dew point, wind barb, cloud cover, weather and pressure.See Wind barb, Surface analysisGeneral usage
Surface analysis: A map of current sea-level pressure, highs, lows and fronts drawn from surface observations, usually every 3 or 6 hours.See Front, Isobar, Station plotGeneral usage
Legend, colour scale: The key that says what each colour or symbol on a map means. On our maps, tapping a legend row highlights only that range on the map.See Potential, index, Calibrated · On the site: ModelMaps, SatelliteThis site
Hatching: Diagonal lines over part of a map that add a second layer of meaning on top of the colours, such as freezing rain on our icing map.See Legend, colour scaleGeneral usage
Loop: A sequence of images played as an animation. Loops show motion, growth and decay that a single image hides.Why it matters. Always loop satellite and radar before deciding anything: a fog patch that is growing and one that is shrinking can look the same in one frame.See Scan, ExtrapolationGeneral usage
Cross-section: A vertical slice through the atmosphere along a line or route, showing cloud, temperature, wind, icing or turbulence by height and distance.See Sounding, FZLVL · On the site: Route briefingGeneral usage
Meteogram: A chart of forecast weather elements over time for one place: temperature, wind, cloud, precipitation, ceiling and visibility in stacked panels.See Forecast hourGeneral usage
Map projection: The method of flattening the curved Earth onto a map. Every projection distorts something: shapes, areas, distances or directions.Why it matters. Distances on northern parts of some maps look much longer than they are.General usage
Hovmöller diagram: A chart with position on one axis (longitude or latitude) and time on the other, coloured by a quantity such as sea-surface temperature anomaly or IVT. Something that moves shows up as a slanted band; something that stays put is a vertical (or horizontal) stripe.Why it matters. It shows motion at a glance: Kelvin waves crossing the Pacific on the ENSO page, atmospheric rivers sliding along the coast on the AR page.See Oceanic Kelvin wave, AR landfall · On the site: ENSO tracker, Atmospheric rivers in BCGeneral usage (after E. Hovmöller, 1949)
Vertical exaggeration: Stretching heights on a 3-D view or cross-section so that the thin atmosphere can be seen: the troposphere is only about 12 km deep over regions hundreds of kilometres wide. At × 14, a 1 km layer is drawn as tall as 14 km of horizontal distance, so slopes look far steeper than they are.See Cross-section · On the site: 3-D atmosphereGeneral usage
Interpolated framesMotion-estimated in-between frames: Frames drawn between two model hours to make an animation smooth. They are estimated by moving the pattern along its measured motion; they are not model output. Values should be read on the model hours themselves.See Loop, Forecast hour · On the site: 3-D atmosphereThis site

Probability and verification

Persistence: A simple forecast: the latest METAR carried forward. A real forecast must beat it to show skill.This site
CSICritical success index: A score for yes-or-no events such as fog. 1 is perfect and 0 is no skill.This site
Frequency bias: How often an event is forecast compared with how often it happens. 1 means as often as it occurs.This site
n: The number of cases behind a score. Fewer cases means the score could easily change.This site
POD, hit rate: Probability of detection: the share of events that were forecast. 1 means none were missed.General usage
FARFalse alarm ratio: The share of forecast events that did not happen. 0 is best.General usage
Brier score: The average squared error of probability forecasts. 0 is perfect. A Brier skill score compares it with a baseline such as climatology; above 0 is better than the baseline.General usage
MAEMean absolute error: The average size of the error, ignoring sign. 2 °C means the forecast was off by 2 degrees on average.General usage
Skill: How much better a forecast does than a simple baseline such as persistence or climatology. A forecast can be accurate but have no skill if the baseline does just as well.General usage
Reliability: Whether forecast probabilities match how often things happen. A reliable 20 % forecast verifies about one time in five.General usage
Verification: Checking forecasts against what was observed, usually METARs, to measure how good they are.General usage
Probability of precipitationPOP: The chance that measurable precipitation falls at a point (or anywhere in an area, depending on the forecast) during a stated period. 40 % means that on days like this, it rains about four times in ten.Why it matters. It isn't a statement about how much of the area or the period will be wet, or how heavy it will be.See What 30 % means, Calibrated, Reliability · Learn: Probability and verification: what 30 % meansGeneral usage
What 30 % means: A 30 % forecast says the event happened on about 3 of every 10 occasions like this one. It's neither a 'no' nor a hedge: on most such occasions nothing happens, but on some it does.Why it matters. For hazards such as thunderstorms or fog at destination, a 30 % chance is often high enough to plan for, which is why TAFs have PROB30.See PROB, Probability of precipitation, Calibrated · Learn: Probability and verification: what 30 % meansGeneral usage
Contingency table: A 2 × 2 table for yes/no forecasts: hits (forecast and happened), misses (happened, not forecast), false alarms (forecast, didn't happen) and correct negatives (neither). Most event scores come from it.See POD, hit rate, FAR, CSI, Frequency bias · Learn: Probability and verification: what 30 % meansGeneral usage
Hits, misses, false alarms: The three cells of a contingency table that involve the event: a hit is a forecast event that happened, a miss is an event nobody forecast, a false alarm is a forecast event that didn't happen.Why it matters. Different users pay different prices for each. A miss of freezing rain is far costlier than a false alarm for most operators.See Contingency table, Cost–loss decisions · Learn: Probability and verification: what 30 % meansGeneral usage
POFDFalse alarm rate: Probability of false detection: of all the times the event did not happen, the share when it was forecast anyway. Not the same as the false alarm ratio, which divides by the number of forecasts.See FAR, POD, hit rate, ROC curveGeneral usage
ETSEquitable threat score: The critical success index adjusted for the hits you'd get by chance. 1 is perfect, 0 is no better than random.See CSI, SkillGeneral usage
HSSHeidke skill score: The share of correct yes/no forecasts after removing those expected by chance, compared with a perfect forecast.See ETS, SkillGeneral usage
RMSERoot mean square error: The square root of the average squared error. Like MAE but it punishes big misses more.See MAE, Mean error, biasGeneral usage
Mean error, bias: The average of forecast minus observed. A temperature bias of +1.5 °C means forecasts run warm on average.Why it matters. A steady bias is the easiest error to fix, and bias correction does exactly that.See Bias correction, MAE, Frequency biasGeneral usage
Reliability diagram: A plot of how often events happened against the probability forecast for them. A perfectly reliable forecast lies on the diagonal: 20 % forecasts verify 20 % of the time, 60 % forecasts 60 % of the time.See Reliability, Calibrated · On the site: Verification · Learn: Probability and verification: what 30 % meansGeneral usage
Sharpness: How often a probability forecast goes out near 0 or 100 % instead of hovering near the average. Sharp and reliable together is the goal.Why it matters. A forecast that always says the climatological 10 % is perfectly reliable but useless for decisions.See Reliability, Climatology · Learn: Probability and verification: what 30 % meansGeneral usage
Discrimination: Whether a forecast gives higher values before events than before non-events. It's what separates a useful forecast from climatology.See ROC curve, SharpnessGeneral usage
ROC curve: A plot of hit rate against false alarm rate as the yes/no threshold on a probability forecast is moved. The area under it measures discrimination: 0.5 is no skill, 1 is perfect.See Discrimination, POFD, POD, hit rateGeneral usage
Base rate: How often an event happens in the sample, for example fog on 4 % of hours at an airport. Also called the climatological frequency.Why it matters. With rare events a forecast that never predicts them can score 96 % correct and be worthless. Good event scores account for the base rate.See Climatology, Rare events · Learn: Probability and verification: what 30 % meansGeneral usage
Rare events: Events such as dense fog, freezing rain or severe hail that happen on a small share of hours. They're hard to forecast and hard to verify because there are few cases.See Base rate, nGeneral usage
Confidence interval: A range around a score that shows how much it could change with a different sample of cases. Small samples give wide intervals.See n, Rare eventsGeneral usage
Forecast uncertainty: How far the real outcome might be from the forecast. It grows with lead time and is larger for small, sensitive things such as fog, convection and freezing drizzle.See Ensemble spread, Predictability, What 30 % means · Learn: Probability and verification: what 30 % meansGeneral usage
Percentile, exceedance: A percentile is the value below which a share of outcomes falls: the 90th percentile gust is exceeded one time in ten. An exceedance probability is the chance of going over a set value.See Ensemble, What 30 % meansGeneral usage
Cost–loss decisions: A simple way to use probabilities: act (pay the cost) when the chance of the event is higher than the cost divided by the loss you'd face if it happened unprotected.Why it matters. It shows why the same 20 % can mean 'carry extra fuel' for one flight and 'carry on' for another.See What 30 % means, Hits, misses, false alarms · Learn: Probability and verification: what 30 % meansGeneral usage
TAF verification: Scoring TAFs against METARs, usually by checking each hour's forecast flight category, ceiling, visibility and wind against what was observed, with rules for TEMPO and PROB groups.See Verification, TAF, Persistence · On the site: VerificationGeneral usage
Climate normals: Averages of weather elements over a standard 30-year period, used as the reference for 'normal'.See Climatology, Base rate · On the site: ClimateGeneral usage
Resolution (of probabilities): How well a set of probability forecasts separates occasions when the event happens from occasions when it does not. Forecasts that always say the climatological frequency are perfectly reliable but have no resolution.Why it matters. A useful forecast must be both reliable and resolved; the Brier score rewards both.See Reliability, Brier score, Sharpness · Learn: Using NWP and AI guidance operationallyGeneral usage
Composite: The average of many past cases that share a feature, for example the mean winter temperature anomaly at an airport over all El Niño winters, compared with all neutral winters.Why it matters. A composite shows a tendency, not what will happen: with about a dozen winters per phase, one unusual winter shifts the mean, and each event differs.See Climatology, El Niño, La Niña, Base rate · On the site: ENSO trackerGeneral usage
Bias (mean error): The average of forecast minus observed over many cases. A wind bias of −2 kt means the forecast was, on average, 2 kt lighter than what the METARs reported; a temperature bias of +1 °C means forecasts ran 1 °C warm.Why it matters. Bias is the error you can correct for in your head: if a model always runs 2 kt light at an airport, add 2 kt. Errors that change sign from day to day (scatter) cannot be corrected that way; MAE and RMSE measure those.See MAE, RMSE, Frequency bias, Verification · On the site: Error and bias mapsGeneral usage (WMO verification practice)
Weather regime: A recurring type of weather situation, such as a clear calm night, post-frontal northwest wind, downslope (chinook) flow or showery convection. Verification split by regime shows how a forecast behaves in each kind of weather rather than on average.Why it matters. A model that is fine on average can be badly wrong in one regime, for example ending radiation fog too early on clear nights. On this site regimes are decided only from what was known when the forecast was issued.See Bias (mean error), Fog regime, Chinook · On the site: Error and bias mapsGeneral usage
Out-of-sample test: Checking a forecast method or a finding on data that was not used to build or tune it, for example training on 2021–2024 and testing on 2024–2026.Why it matters. Any method looks good on the data it was fitted to. Only an out-of-sample result says whether it will work on tomorrow's weather.See Verification, Two-period check, Skill · On the site: Airport intelligenceGeneral usage
Two-period checkdiscovery and confirmation: Splitting a record into two separate periods (here before and after July 2024) and keeping a finding only if it shows up in both. The first period discovers it, the second confirms it.Why it matters. With many airports, patterns and scores, some 'findings' are luck. Requiring the same result in two separate periods throws most of the luck out.See Out-of-sample test, 95 % range (moving-block bootstrap) · On the site: Airport intelligenceGeneral usage
95 % range (moving-block bootstrap): An uncertainty range made by re-sampling the record many times in blocks of consecutive days (here 5-day blocks) and seeing how much the result changes. Blocks keep together days whose weather, and forecast errors, are linked.Why it matters. Hourly forecast errors on the same day are not independent; treating every hour as a separate case makes ranges far too narrow and findings look surer than they are.See Two-period check, Out-of-sample test · On the site: Airport intelligenceKünsch 1989; general usage
EOFEmpirical orthogonal function (principal component): A statistical way of finding the few patterns that explain most of the variation in a weather field over many days. Each EOF is a map pattern; each day gets a score for how strongly it shows that pattern. The PNA and other teleconnection patterns are found this way.Why it matters. Comparing days by their scores on the leading EOFs is a fast, noise-resistant way to find analog days.See Atmospheric fingerprint, Analogs, Pacific–North American pattern, Pattern correlationGeneral usage (Wilks, Statistical Methods in the Atmospheric Sciences)
Pattern correlation: A number from −1 to +1 that says how alike two maps are in shape (for example two 500 hPa anomaly maps), regardless of how strong they are. Above about 0.6 the patterns are clearly similar.See Anomaly, Analogs, Atmospheric fingerprintGeneral usage
Forecast bust: A forecast that went clearly wrong on something that mattered: fog that formed when none was forecast, IFR that never came, freezing rain that fell as snow. Studying busts (what the forecast assumed and what the observations showed) is how forecasters and models improve.See Verification, FAR, Hits, misses, false alarms · On the site: Weather DetectiveGeneral usage

On this site

Updated, STALE: The strip under the menu shows how old the product on screen is. STALE means it's older than the limit for that product, so a newer version should exist by now. Amber means something else to check, such as an older run being shown.This site
TAFbuilder: Our tool for working with automated TAF-style forecasts.This site
ModelMaps: Our map viewer for model forecasts, by model, run and forecast hour.This site
Screening product: A product that points to where something might be happening, to look at more closely. It is not an observation or a forecast of that thing.This site
Potential, index: Words on our maps for scores that rank how favourable conditions are. They are not probabilities unless the legend says calibrated.This site
Experimental: A product still being tested and compared with observations. Use it as extra information, never on its own.This site
Fog model (this site): Our physical simulation of the lowest 1.5 km of air over an airport, run hour by hour 11 times from slightly different starting conditions, shown on the airport page.Why it matters. It sees only the air above the airport, so it can over-forecast fog when cloud or drier air is on the way. Read it with the fog discussion.See Support index, Radiation fog, Ensemble · On the site: Airport page · Learn: Fog: types and life cycleThis site
Fog, wind and ceiling discussions: Automatic forecaster-style write-ups on the airport page, listing when conditions may change, chances by hour and the reasons for and against. Each reason has a small ? that explains it.Why it matters. They are written by AI from the data, are unverified and are not an issued forecast.See Fog model (this site), AI, learned · On the site: Airport pageThis site
TAF-style text: Our guidance written in TAF code following MANAIR coding rules, so it can be compared line by line with the issued TAF. It's never a substitute for the official TAF.See TAF, TAFbuilder · On the site: Airport page, TAFbuilder · Learn: Reading a METAR and a TAFThis site
Upstream reports: METARs from stations the low-level wind is carrying air from, with an estimate of how long that air takes to arrive at the airport.Why it matters. Fog or low cloud upstream is an early warning, though air changes on the way, especially over terrain.See Advection fog, Trend · On the site: Airport pageThis site
FRONTIER hour table: One row per hour with calibrated chances of a ceiling below 3,000 ft, visibility below 6 SM, fog, precipitation, freezing precipitation, gusts of 25 KT or more and thunderstorms.See FRONTIER, Calibrated · On the site: FRONTIER · Learn: Probability and verification: what 30 % meansThis site
Dashboard: A page with your chosen airports side by side: the latest METAR, hazards and the flight category by hour.See Category colours · On the site: DashboardThis site
Route briefing page: Our page for a flight between two airports: weather at each end and a 2-D cross-section of the route with cloud, icing, turbulence and winds by height. Model guidance, not an official briefing.See Weather briefing, Cross-section · On the site: Route briefingThis site
Airport climate page: Climatology from the METAR record: how often each condition happens at an airport, by month and hour.See Climatology, Base rate · On the site: ClimateThis site
Learn: Our short lessons on aviation weather, each with diagrams and a few self-check questions, written in our own words.On the site: LearnThis site
Turbulence potential: Our maps of how unusual the model's turbulence ingredients are at a level compared with its own past. LGT, MOD, SEV and EXT mark the top 5 %, 1 %, 0.1 % and 0.01 % and are experimental, not yet calibrated against PIREPs.See Clear-air turbulence, Light, moderate, severe, extreme, EDR · On the site: ModelMaps: Turbulence potentialThis site
Altitude of maximum turbulence: The flight level where our turbulence signal is strongest, shown where it reaches at least the light category. Experimental.See Turbulence potential · On the site: ModelMaps: Altitude of maximum turbulenceThis site
Atmospheric fingerprint: MeteoFrontier's compact description of the large-scale weather around a region on one day: the shape of the pressure pattern and how warm, moist, stable and windy the air is, all as departures from normal for the date. Two days with similar fingerprints are analogs.See Analogs, Anomaly, EOF, Pattern correlation · On the site: Atmospheric fingerprintThis site

Spotted something off? Trust the official manuals: this page is a plain-language aid.