Meteorology
Meteorology tests how the atmosphere produces weather and how a pilot decodes forecasts and reports for a go/no-go decision.
Pressure and altimetry
ISA: sea level 15°C, 1013.25 hPa, lapse rate 1.98°C/1000 ft. QNH gives altitude above sea level; QFE gives height above the reference point. Below transition altitude set QNH; above set 1013.2. Cold air makes the altimeter over-read; flying from high to low pressure also makes it over-read unless corrected.
Stability, cloud and hazards
Stable air gives stratiform cloud, drizzle, mist and smooth air; unstable air gives cumuliform cloud, showers, turbulence and thunderstorms. Lapse rate versus dewpoint determines cloud base. Fog forms when temperature reaches dewpoint; radiation fog is favoured by clear skies, light wind and high humidity. Airframe icing occurs near 0°C in visible moisture; carburettor ice can occur at higher temperatures. Thunderstorms bring gust fronts, wind shear, hail, lightning and microbursts. Warm fronts give gradual cloud and rain; cold fronts give a narrow band of heavy showers.
Reports
METAR is an observed report, usually issued hourly; TAF is a forecast. Decode wind, visibility, cloud, temperature/dewpoint, QNH and trend.
Worked example
METAR LLBG 121220Z 32012KT 9999 SCT025 24/18 Q1013: on the 12th at 12:20 UTC, wind 320° at 12 kt, visibility 10 km or more, scattered cloud at 2,500 ft, temperature 24°C, dewpoint 18°C, QNH 1013 hPa. The 6°C spread and scattered cumulus suggest moderate humidity and convective cloud, not fog.
Traps
Confusing QNH with QFE; reading wind direction as 'to' rather than 'from'; ignoring temperature in TAF; and treating a forecast as an observation.