A METAR is a routine weather observation from an airport, written in a compact code that pilots and dispatchers read the same way everywhere. It looks cryptic at first, but the groups always come in the same order, so once you know them you can read any METAR from left to right.
A first example
EPWR 291830Z 24008KT 9999 SCT030 15/09 Q1018
EPWR
Station: the ICAO code of the airport (here Wrocław).
291830Z
Day 29 of the month, 18:30 UTC. The Z means Zulu time, which is UTC.
24008KT
Wind from 240 degrees at 8 knots.
9999
Visibility 10 km or more.
SCT030
Scattered cloud (3 to 4 eighths of the sky) at 3,000 ft above the airport.
15/09
Temperature 15 °C, dew point 9 °C.
Q1018
QNH (altimeter setting) 1018 hPa.
This is an illustrative report, not a live one. You can paste it into the decoder or open a real one on any airport page.
The groups, in order
Report type. METAR (routine) or SPECI (special report). It may be followed by COR (a correction) or AUTO (fully automatic, no human observer).
Station. The four-letter ICAO code of the airport.
Date and time. DDHHMMZ: day, hour and minute in UTC.
Wind. Direction in degrees true, then speed, then the unit (KT, MPS or KMH). A G adds the gust speed (24015G25KT). VRB means variable direction and 00000KT means calm. A group such as 190V250 gives the range when the direction is varying. Note that METAR wind direction is relative to true north, not magnetic.
Visibility. Metres as four digits (9999 means 10 km or more), or statute miles in the US and Canada (10SM). CAVOK replaces the visibility, weather and cloud groups when conditions are good: visibility 10 km or more, no significant cloud, no significant weather.
Runway visual range (only sometimes). R27/0600 means 600 m on runway 27.
Present weather. An optional intensity (- light, + heavy), then codes such as RA (rain), SN (snow), FG (fog), BR (mist) or TS (thunderstorm). -RA is light rain, +TSRA is a thunderstorm with heavy rain.
Cloud. FEW, SCT, BKN or OVC followed by the height in hundreds of feet above the airport. CB marks cumulonimbus. NSC, SKC or NCD mean no significant cloud, or sky clear. VV means the sky is obscured.
Temperature and dew point in degrees Celsius. An M means below zero (M02 is minus 2).
Pressure. Q followed by hPa (Q1018), or A followed by inches of mercury in hundredths (A3002 is 30.02 inHg). See QNH and altimeter settings.
Optional extras. Recent weather (RE), wind shear (WS), a trend forecast (NOSIG, BECMG, TEMPO) and remarks (RMK).
A second example, with weather
EGLL 011320Z 22015G27KT 190V250 6000 -RA BKN012 OVC025 12/10 Q0998 TEMPO 3000 RA
Wind from 220 degrees at 15 kt, gusting 27 kt, with the direction varying between 190 and 250 degrees.
Visibility 6,000 m (6 km) in light rain (-RA).
Broken cloud at 1,200 ft and overcast at 2,500 ft. The lowest broken or overcast layer is the ceiling, here 1,200 ft.
Temperature 12 °C, dew point 10 °C: a spread of only 2 degrees, so the air is nearly saturated.
US reports use statute miles for visibility (10SM) and inches of mercury for pressure (A3002 is 30.02 inHg). They also add a remarks section after RMK: AO2 identifies an automated station with a precipitation sensor, SLP165 is the sea-level pressure (1016.5 hPa) and the T group gives the temperature and dew point to a tenth of a degree. Everything before RMK follows the same logic as the European examples.
Practical tips
A temperature and dew point within about 3 °C of each other, especially on a calm evening or morning, points to fog or low cloud.
The ceiling decides a lot: it is the lowest BKN or OVC layer (or VV). See flight categories.
A METAR describes now. For what is coming, read the TAF.