• METAR: current
  • TAF: forcast


METAR is a format for reporting weather information.

  • Wind: direction from which the wind is blowing + speed.
    • E.g. 12015G20KT: wind from 120 degree, 15 knots, G gusting 20 knots
  • Visibility: in statue miles e.g. 5SM
  • Weather: -RA=light rain +RA=heavy rain, BR=mist(from French word "brume")
  • Cloud: SCT048 = scattered cloud layer at 4800' AGL, BKN065: broken
  • 150V210=variable wind between 150 degree and 210 degree
  • VRB005KT=Wind direction is vairable, speed is less than 6 kts
  • OVC008CB, CB=Cumulonimbus
  • VV008: sky obscured with a vertical visibility of 800'.
  • RAB35 means that "rain began at 35 minutes past the hour".


  • AO1: weather sensor that cannot discriminate between freezing and non-freezing precipitation.
  • A02: the site is automated and HAS a precipitation sensor; can discriminate between freezing and non-freezing precipitation.
  • TSE36: Thunderstorm ended 36 minutes past the hour
  • PRESFR: Pressure is falling rapidly
  • SLP – Sea Level Pressure
  • RAESNB42M: Rain ended and snow began 42 minutes past the hour.

If there is an AUTO after the ID in the metar ob, then there is no observer (un-manned).


  • UA: Routine report.
  • UUA: urgent upper air report.
  • OV: location ("over"?)
  • TM: time.
  • FL: flight level, altitude
  • TP: aircraft type.
  • SK: sky cover.
  • WX: weather.
  • TA: temperature (Celsius)
  • WV: wind
  • TB: turbulence.
  • IC: icing.
  • RM: Remarks.
  • RM +/- 15 KTS ON FINAL indicates a remark reporting a plus or minus 15 knot variation of airspeed on final approach.


  • BL: between layers

Winds and Temperature Aloft Forecast (FD)

If a wind direction is coded between 51 and 86, the wind speed is 100 knots or greater. To decode, subtract 50 from the direction and add 100 knots to the speed. "830558" is 83 - 50 = 33(0), 05 + 100 = 105 knots, and a -58° C temperature.


ATIS: Automatic terminal information service, broadcasts essential information like weather, active runways, available approaches, etc.

  • (Voice) ATIS: tune in the frequency to listen.
  • D-ATIS: Data-link ATIS, is text-based, can be displayed on an electronic display in the aircraft.

The ceiling/sky condition, visibility, and obstructions to vision may be omitted from the ATIS broadcast if the ceiling is above 5,000' and the visibility is more than 5 miles.


  • ATIS: Automatic Terminal Information Service. contains current, routine information to arriving and departing aircraft as well as weather reports derived from human data collection that is updated hourly or upon pertinent data changes.
  • AWOS: Automated Weather Observing System. Typically includes ceiling and sky conditions, visibility, temperature, dew point, altimeter setting and wind speed, gusts and direction.
  • ASOS: Automated Surface Observing System. Can additionally provide the type and intensity of precipitation (rain, snow, freezing rain), and obstructions to visibility such as fog and haze.


  • ATIS is by human recording. AWOS and ASOS are automated.
  • ATIS is generated hourly (or as required); AWOS is minute-by-minute.
  • ASOS is similar to AWOS but is managed, maintained and controlled by NOAA rather than the FAA or the organization that operates the airport.


SIGMET (Significant Meteorological Information)

AIRMETs cover less severe weather: moderate turbulence and icing, sustained surface winds of 30 knots or more, or widespread restricted visibility. SIGMETs cover more severe weather.

3 types of AIRMET

  • S (Sierra)
  • T (Tango)
  • Z (Zulu)

3 main types of SIGMET:

  • Volcanic ash (VA or WV SIGMET)
  • Tropical Cyclone (TC SIGMET)
  • Other En-route weather phenomenon (WS SIGMET)

AIRMETs are valid for six hours. SIGMET valid up to four hours. SIGMETS for hurricanes and volcanic ash outside the CONUS are valid up to six hours.

Both broadcast on the ATIS at ATC facilities, as well as over VOLMET stations


  • PIREP in MSL
  • METAR in AGL

Ice pellets and hail

Thunderstorms may create hail rather than ice pellets.

Rain falling through colder air may freeze during its descent falling as ice pellets. Ice pellets always indicate freezing rain at higher altitudes.

Update frequency

  • METAR: update every hour
  • TAF: update every 6 hour, each has a 24 hour forecast period


  • Radiation fog does not form over water. Under stable nighttime conditions, long-wave radiation is emitted by the ground; this cools the ground, which causes a temperature inversion.
  • Advection fog forms when moist air moves over colder ground or water. It deepens as the wind speed increases up to 15 knots.
  • Upslope fog: occurs when sloping terrain lifts air, cooling it adiabatically to its dew point and saturation. Upslope winds more than 10 to 12 knots usually result in stratus rather than fog.
  • Steam fog (arctic sea smoke): In northern latitudes, steam fog forms when water vapor is added to air that is much colder, then condenses into fog. Steam fog forms in winter when cold, dry air passes over comparatively warm water. The result is low level turbulence and icing.
  • Frontal fog. Associated with frontal zones and frontal passages.
    • warm-front pre-frontal fog;
    • cold front post-frontal fog;
    • frontal-passage fog.
  • Ice fog. Ice fog is composed of ice crystals instead of water droplets and forms in extremely cold, arctic air

Both advection fog and upslope fog depend on the movement of wind to form.



  • single-cell
  • multi-cell
  • squall line
  • supercell (capable of producing tornados)

Formation of thunderstorms: Sufficient moisture, an unstable lapse rate, and a lifting action.


幡状云(学名:Virga,缩写: vir )是一种从云中落下的降水,但还没到地面前就已经蒸发。


  • advection 平流
  • convection 对流



  • +: Heavy
  • -: Light
  • VC: Vicinity
  • MI Shallow (MI = MInce, French origin)
  • PR Partial
  • BC Patches (BC = BanCs, French origin)
  • DR Low Drifting
  • BL Blowing
  • SH Shower(s)
  • TS Thunderstorms
  • TSRA: Thunderstorm with Rain
  • FZ Freezing
  • WDSPR: Widespread.


  • DZ Drizzle
  • RA Rain
  • SN Snow
  • SG Snow Grains
  • IC Ice Crystals (Diamond Dust)
  • PL Ice Pellets
  • GR Hail
  • GS Snow Pellets
  • UP Unknown Precipitation


  • BR Mist
  • FG Fog
  • FU Smoke
  • VA Volcanic Ash
  • DU Widespread Dust
  • SA Sand
  • HZ Haze
  • PY Spray
  • DRDU: Drifting Dust
  • DRSA: Drifting Sand

Other Weather

  • PO Dust/Sand Whirls
  • SQ Squalls
  • FC Funnel Cloud, Tornado or Waterspout
  • SS Sandstorm
  • DS Dust Storm


  • CIG: Ceiling.

Weather Services

GFA: Graphical Forecasts for Aviation

NEXRAD: Next Generation Weather Radar. Introduced in 1988 ("Next" does not mean very new or recent...); operated by National Weather Service (NWS).

Geostrophic wind

Geostrophic wind blows parallel to the isobars because the Coriolis force and pressure gradient force are in balance.


  • High Clouds 16,500-50,000 ft
    • Cirrus, almost entirely ice crystals
    • cirrostratus: high-level layer stable layer
  • Middle Clouds: 6,500 - 23,000 ft, liquid water, named with prefix "alto-"
    • Altocumulus: Cumuloform clouds that form in the middle part of the troposphere
    • altostratus
    • nimbostratus
  • Low level: ground to 6,500 ft
    • Stratus
    • Cumulus
    • Stratocumulus
  • vertically Developed
    • towering cumulus
    • cumulonimbus


One of the two conditions for structural icing to form is the aircraft must be flying through visible moisture.


Frost forms in much the same way as dew. The difference is that the collecting surface must be colder than the dew point of the surrounding air and the dew point must be colder than freezing.

Temperature Inversion

The most frequent type of ground or surface-based temperature inversion is that which is produced by: The ground radiates and cools much faster than the overlying air. The air in contact with the ground becomes cold while the temperature a few hundred feet above changes very little. Thus, temperature increases with altitude.

Inversions commonly occur within the lowest few thousand feet above ground. Strong wind shears often occur across temperature inversion layers, which can generate turbulence.

A pilot can expect a wind-shear zone in a temperature inversion whenever the windspeed at 2,000 to 4,000 feet above the surface is at least 25 knots or more.


2635-08 indicates the wind from 260° true at 35 knots, with a temperature of -8° Celsius.

What to expect around ...

  • Trough (an elongated region of low atmospheric pressure): often result in increased cloud cover and precipitation. Rain, snow, or even thunderstorms can develop along and near the trough axis.
    • Positively Tilted Trough: southwest to northeast, a weakening weather system, less likely to result in severe weather.
    • Negatively Tilted Trough: northwest to southeast, more likely to produce severe weather.
  • Ridge: expect fair weather.
  • Cold Front:
    • Cold fronts commonly follow closely behind a trough.
    • Rain and even thunderstorms can form as the moisture in the warm air mass rises, cools, and condenses.
  • Warm Front:
    • expect light rain.

Rain vs Drizzles

Both rain and drizzle are types of precipitation. The only difference is the size of the drops. Drizzle drops, which come from low-lying clouds like stratus, are smaller and fall a lot slower. Raindrops, on the other hand, are bigger and fall at a faster speed.

Frontolysis vs Frontogenesis

  • Frontogenesis: the initial formation of a frontal zone,
  • Frontolysis: the dissipation or weakening of an atmospheric front.


Special code:

  • 9900: the forecast speed is less than 5 knots, read "LIGHT AND VARIABLE".
  • 9999: used in some international METARs and TAFs to indicate that visibilities are 10 KM or above. This is not used in US METARs or - TAFs.
  • 0000: used in some international METARs and TAFs to indicate that visibilities are less than 50 meters. This is not used in US METARs or TAFs.

Forecast wind speeds of 100 through 199 knots are indicated by subtracting 100 from the speed and adding 50 to the coded direction. e.g.

  • 7545: wind from 250 degrees at 145 knots.

Above 24,000 feet, the sign is omitted since temperatures are always negative at those altitudes. For wind >=200 kt, use 199, e.g. 7799 means wind from 270, speed >= 199 kt.