Aviation Weather Information: METAR, TAF, SIGMET, Charts and Briefings
METAR and TAF
A METAR reports observed conditions: wind direction and speed with gusts, visibility, present weather, cloud amount and base, temperature and dew point, QNH and trend. Cloud amounts are FEW (1–2 oktas), SCT (3–4), BKN (5–7) and OVC (8). BKN or OVC at the lowest layer defines the ceiling. A TAF gives a forecast with validity period, and change groups TEMPO for temporary fluctuations, BECMG for gradual change, and FM for a rapid change.
Warnings and charts
SIGMET warns of significant weather such as severe turbulence, icing, thunderstorms, tropical cyclones and volcanic ash. AIRMET covers less severe phenomena. Significant weather charts show jet streams, tropopause height, cumulonimbus and turbulence areas. Wind and temperature charts give upper winds for flight planning.
Traps
A common trap is reading CAVOK as simply good visibility; it also means no cloud below 5000 ft, no cumulonimbus and no significant weather. Another is confusing TEMPO (temporary, less than an hour) with BECMG (permanent change within the period). Remember that a SPECI is a special report issued on significant change, not a routine report.
Worked example
METAR: wind 240/12 kt, visibility 8000 m, BKN 1200 ft, temperature 27°C, dew point 24°C, QNH 1009. The spread is 3°C, so expect a low cloud base near 1200 ft and a risk of showers, consistent with the BKN layer.