A METAR fits on one line, and every group has a fixed place in it. Read from left to right, it gives the time, the wind, the visibility, the present weather, the clouds, the temperature and the pressure — always in that order. Here is a real report from Dublin, and what each group means.
The header: who, when
METAR EIDW 101030Z: the message type, the ICAO indicator of the aerodrome, then the day of the month and the time in UTC — the 10th, at 10:30. A SPECI instead of METAR is a special observation, issued between two routine reports because the weather changed markedly.
AUTO after the time marks an automatic station, with no observer: some groups are missing or written ///, and the clouds may read NCD, "no cloud detected".
METAR EIDW 101030Z 21015KT 160V240 9999 -RA FEW017 SCT020 BKN022 18/14 Q1007 NOSIG
The wind
21015KT: the direction the wind comes FROM, in true degrees, on three digits; the speed on two or three; the unit, knots here, sometimes MPS for metres per second. G introduces gusts: 21015G27KT, fifteen knots on average, twenty-seven in gusts.
VRB03KT: variable, too light to have a direction. 00000KT: calm. A second group, 160V240, says the direction varied between 160 and 240 degrees during the ten minutes of the observation.
The direction is true; the number painted on a runway is magnetic: laying one on the other takes a calculation, the one on the crosswind page.
The visibility
9999: ten kilometres or more. 4000: four thousand metres; 0800: eight hundred. In the United States it is counted in statute miles: 10SM, 1 1/2SM, and P6SM for "more than six".
CAVOK replaces the visibility, the present weather and the clouds at once: visibility of at least ten kilometres, no cloud below five thousand feet and no cumulonimbus, no significant weather.
A minimum visibility may follow with its direction, 2000 1200NW; the runway visual range reads R27L/0550N: on runway 27 left, five hundred and fifty metres, no change.
The present weather
-RA: light rain. The sign gives the intensity — - light, nothing for moderate, + heavy — and VC means "in the vicinity", between eight and sixteen kilometres from the aerodrome.
A descriptor may precede the phenomenon: SH showers, TS thunderstorm, FZ freezing, BL blowing, MI shallow, BC patches. TSRA is a thunderstorm with rain; FZFG a freezing fog.
The most frequent phenomena: RA rain, DZ drizzle, SN snow, GR hail, GS small hail, FG fog — visibility below one thousand metres —, BR mist — from one to five thousand —, HZ haze, FU smoke.
The clouds
FEW017 SCT020 BKN022: three layers, each by its amount and the height of its base in hundreds of feet above the aerodrome. FEW covers one to two oktas — eighths of the sky —, SCT three to four, BKN five to seven, OVC all eight. 017: one thousand seven hundred feet.
CB or TCU after the height names a cumulonimbus or a towering cumulus. NSC: no significant cloud; VV002: the sky is obscured, and the vertical visibility is two hundred feet; ///: not measured.
The ceiling is the base of the first BKN or OVC layer, or the vertical visibility. It is what classifies the report into a flight category, together with the visibility.
Temperature, dew point, QNH
18/14: eighteen degrees, dew point fourteen. An M marks a negative value: M02/M05. A small spread between the two announces air close to saturation — mist, fog, low cloud.
Q1007: the QNH in hectopascals, the pressure reduced to sea level that you set on the altimeter to read an altitude. In the United States, A2992: twenty-nine point ninety-two inches of mercury.
The trend, and the end
NOSIG: no significant change expected within two hours. BECMG announces a lasting change, TEMPO temporary fluctuations, each followed by the groups that change — TEMPO 4000 RA. The report sometimes ends with remarks, RMK, specific to each country, then with =.
The site's decoder reads all of this group by group, in English, and draws the summary: the wind rose on the runway plan, the cloud profile, the visibility, the temperature, the QNH.
What a METAR does not say
It is an observation, at the time of its time group, renewed every thirty minutes or every hour: it forecasts nothing, that is the TAF's job. It does not say which runway is in use — that is the ATIS —, nor what a visibility is worth for YOUR flight: the minima are those of the regulation and of your licence.