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 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.