Meteorology
Meteorology tests the atmosphere as it affects flight: pressure and altimetry, temperature and stability, wind, cloud, precipitation, icing, thunderstorms, fronts, and the interpretation of METAR, TAF and significant weather charts.
Atmosphere and altimetry
ISA is 15°C at MSL with a 2°C per 1,000 ft lapse and 1013.25 hPa. An altimeter set to QNH reads elevation on the ground; set to 1013 it reads pressure altitude. A 1 hPa error is about 30 ft. Flying from high to low pressure, or warm to cold, the true altitude is lower than indicated - a classic terrain risk.
Stability and cloud
Warm air over a warm surface is unstable; cooling from below stabilises. Cumuliform cloud means instability and turbulence; stratiform cloud means stable air. Orographic lift, fronts and convection all produce lift.
Icing and thunderstorms
Airframe icing needs visible moisture and temperatures roughly between 0 and −20°C; carburettor icing can occur well above freezing. A thunderstorm has three stages, and the mature stage brings hail, severe turbulence, lightning and microbursts. Avoid by at least 20 nm.
Worked example
Surface temperature 20°C, dewpoint 12°C. The spread is 8°C; with a lapse of 2°C per 1,000 ft, saturation occurs at about 4,000 ft, so convective cloud base is roughly 3,000–4,000 ft AGL (the 400 ft per °C rule gives 3,200 ft).
METAR and TAF
Read wind, visibility, weather, cloud, temperature and QNH. TEMPO means temporary fluctuation; BECMG means gradual change.
Traps
Using QFE values as QNH; forgetting that METAR and TAF winds are true while ATIS wind is magnetic; and misreading cloud amounts: FEW 1–2 oktas, SCT 3–4, BKN 5–7, OVC 8.