A METAR is a standardized surface weather observation, transmitted by an airport station at least once an hour, and immediately when conditions change sharply. The instruments are calibrated and maintained to aviation standards, because pilots make decisions with them.
Here's a real one, taken apart:
Flying Cloud airport, on the 14th at 20:53 UTC. Wind from 120° at 10 knots. Visibility 7 statute miles. A few clouds at 3,500 feet, overcast at 5,500. Temperature 26°C, dew point 18°C. Pressure 30.06 inches.
Piece by piece
- KFCM: the station identifier. US stations start with K.
- 142053Z: day of month, then time in UTC (the Z). The 14th at 20:53 Zulu.
- 12010KT: wind direction and speed: from 120 degrees at 10 knots. A G means gusts (12010G18KT). VRB means variable.
- 7SM: visibility in statute miles. 10SM is the maximum they report and means "clear as far as we measure."
- FEW035 OVC055: cloud layers and their heights in hundreds of feet. FEW, SCT (scattered), BKN (broken), OVC (overcast). SKC or CLR means nothing up there.
- 26/18: temperature and dew point in Celsius. An M prefix means minus (M02/M05).
- A3006: altimeter setting: 30.06 inches of mercury.
- RMK: everything after this is remarks: sea-level pressure, precise temperatures, pressure tendency, and other notes.
Weather codes
When something is actually falling or obscuring, it appears before the clouds: RA rain, SN snow, TS thunderstorm, BR mist, FG fog, HZ haze, DZ drizzle. Intensity is marked with a minus for light or a plus for heavy: -RA is light rain, +TSRA is a heavy thunderstorm with rain. VC means "in the vicinity."
SPECI: when the station interrupts
Stations normally report on the hour. When conditions change fast enough to matter, visibility collapses, a thunderstorm begins, wind shifts hard, the station transmits an unscheduled SPECI report instead of waiting. A special report is the station saying something is happening right now, which no model-derived forecast can tell you.
Why bother
Because it's the raw material. Every pretty weather interface you've ever used, including the ones with subscription tiers, is a rendering of data like this, usually after being blended, interpolated, and smoothed. Reading the source means you see what was actually measured, when, and by which instrument, and you can tell the difference between a forecast and a fact.