Weather Reports and Forecasts

A METAR is an airport’s routine weather report, issued every half hour or hour at its own minutes past the hour. A SPECI is a special report issued between two of them, when conditions change significantly. Type WX and an airport code in the console for the latest, or look at the route manager. Point at any group to read it.

Each country reports in its own way. Flight Dispatch shows every report and forecast as it was sent.

Reading a METAR

METAR EGLL 141020Z 24012G24KT 210V270 6000 -RA FEW008 BKN030 SCT045CB 14/12 Q1018 NOSIG

METAR EGLL 141020Z: A routine report from Heathrow, observed on the 14th at 1020Z. SPECI is a special report, COR a correction to one sent earlier, and AUTO after the time one made by instruments alone.
24012G24KT 210V270: Wind from 240° true at 12 knots, gusting 24, its direction varying between 210° and 270°. A gust is given when it is 10 knots or more over the mean. 00000KT is calm, VRB a light wind with no direction to speak of, and MPS metres per second.
6000: Prevailing visibility 6,000 m. 9999 is 10 km or more. A second group with a direction, 1200NE, is the lowest visibility where one direction is much worse.
R27R/0600D: Runway visual range, given when visibility is under 1,500 m: 600 m on runway 27R and falling. U is improving, N steady; P2000 more than 2,000 m and M0050 less than 50 m.
-RA: Light rain. - is light, + heavy, and no sign moderate. See the weather codes below.
FEW008 BKN030 SCT045CB: A few clouds at 800 ft, broken cloud at 3,000 ft and scattered cumulonimbus at 4,500 ft, all above the airfield. FEW is 1 to 2 eighths of the sky, SCT 3 to 4, BKN 5 to 7 and OVC all of it. CB is cumulonimbus and TCU towering cumulus. NSC is no significant cloud, and VV002 a sky hidden by fog or snow, with 200 ft of vertical visibility.
CAVOK: In place of the visibility, weather and cloud: 10 km or more, no cloud below 5,000 ft, no cumulonimbus or towering cumulus, and no significant weather.
14/12: Temperature 14°C, dew point 12°C. M is minus: M02/M05.
Q1018: QNH 1018 hPa, rounded down.
RERA, WS R27R: After the pressure: recent weather (rain, since the last report but not now), and wind shear reported on a runway (WS ALL RWY for all of them).

Weather codes

DZ RA SN SG PL GR GS: Drizzle, rain, snow, snow grains, ice pellets, hail and small hail. Two together are both falling, the heavier first: RASN.
FG BR HZ: Fog (visibility under 1,000 m), mist (1,000 to 5,000 m) and haze.
SH TS FZ: Before what falls: showers, thunderstorm, freezing. +TSRAGR is a thunderstorm with heavy rain and hail; FZFG freezing fog; TS alone a thunderstorm with nothing falling.
MI BC PR: Before fog: shallow, in patches, over part of the airfield.

Automatic stations

After AUTO, instruments report what they can. NDV after the visibility means no direction can be told. /// after a cloud layer means its type can’t be told, and NCD that no cloud was detected. Slashes in place of a whole group (/////KT, ////, Q////) mean that sensor is out of service.

TREND

Larger airports end a report with a forecast for the next two hours.

NOSIG: No significant change expected.
BECMG FM1100 TL1200 3000 RA BKN008: Becoming, between 1100Z and 1200Z, visibility 3,000 m in rain with broken cloud at 800 ft, and staying so. AT gives a single time.
TEMPO TL1230 30020G35KT: At times until 1230Z, wind 300° at 20 knots gusting 35, for under an hour at a time.
NSW: No significant weather: what there was is expected to end.

TAF

A TAF is the forecast for an airport: 30 hours ahead at the largest, 24 at most, and 9 at small ones, issued every six hours (every three for a 9-hour TAF), an hour before its period starts. It gives the wind, visibility, weather and cloud expected to prevail, in the same groups as a METAR, then what’s expected to change.

TAF EGLL 141100Z 1412/1518 24012KT 9999 SCT030
      BECMG 1418/1420 28015G25KT
      TEMPO 1420/1502 4000 SHRA BKN012 SCT020CB
      PROB30 TEMPO 1504/1508 0500 FG VV002
      FM150900 31008KT CAVOK

141100Z 1412/1518: Issued on the 14th at 1100Z, for 1200Z on the 14th to 1800Z on the 15th. The end of a day is written as hour 24: 1500/1524.
BECMG 1418/1420: Becoming, between 1800Z and 2000Z: a lasting change, given with only what changes.
TEMPO 1420/1502: At times between 2000Z and 0200Z, for under an hour at a time and less than half the period in all.
PROB30, PROB40: A 30% or 40% chance, alone or with TEMPO. Less likely than that isn’t forecast, and more likely is given as TEMPO or BECMG.
FM150900: From 0900Z on the 15th, conditions change to those that follow, all given afresh.
TAF AMD: An amended forecast, replacing the one in force for the rest of its period. One is issued when what’s observed has gone outside what the forecast allowed for.

North American TAFs give visibility in statute miles, with P6SM for more than six, use FM for every lasting change and never BECMG, and of the probabilities only PROB30.

North American reports

METAR KJFK 141051Z 26012G22KT 1 1/2SM R04L/2400FT +TSRA BR FEW008 BKN025CB OVC075 22/21 A2993 RMK AO2 SLP139 T02220211

1 1/2SM: Visibility in statute miles. 10SM is 10 miles or more; M1/4SM less than a quarter.
R04L/2400FT: Runway visual range in feet.
CLR, SKC: No cloud below 12,000 ft, from instruments; sky clear, from an observer. CAVOK isn’t used. Every layer is reported, not only those below 5,000 ft.
A2993: Altimeter setting 29.93 inches of mercury.
RMK AO2 SLP139 T02220211: Remarks. AO2: an automated station that can tell rain from snow. SLP139: sea-level pressure 1013.9 hPa. T02220211: temperature 22.2°C and dew point 21.1°C; a 1 in place of the first 0 of either is minus. Canada gives each layer’s cloud type and eighths instead, SC4AC2.

Reports are hourly, a few minutes before the hour in the United States and on the hour in Canada, with SPECIs between them.

When a SPECI is issued

Stations that report hourly issue a SPECI when, since their last report:

the wind changes direction by 60° or more with 10 knots or more, or its speed changes by 10 knots;
visibility passes 800, 1,500, 3,000 or 5,000 m, or runway visual range 175, 300, 550 or 800 m;
a thunderstorm, hail, fog, freezing precipitation, or moderate or heavy precipitation begins or ends;
the lowest broken or overcast layer passes 100, 200, 500, 1,000 or 1,500 ft.

A change for the worse is reported at once, and one for the better when it has lasted ten minutes. Stations that report every half hour mostly issue none.