Meteorology and Altimetry
What the paper tests
This domain covers the atmosphere the aircraft operates in: the ISA, altimetry and pressure settings, cloud and icing, thunderstorms and wind shear, jet streams and tropopause behaviour, and decoding METAR, SPECI and TAF reports for operational decisions.
Core rules
- ISA: sea-level pressure 1013.25 hPa, temperature 15 °C, lapse rate 1.98 °C per 1,000 ft up to the tropopause near 36,000 ft.
- QNH set gives aerodrome elevation on the ground; QFE gives zero on the ground; 1013.2 set above the transition altitude gives flight levels.
- The pressure rule: roughly 30 ft of altitude per hPa near sea level.
- Cold air makes true altitude lower than indicated. 'High to low, look out below' remains the most examinable sentence in the subject.
- Icing risk is greatest in the 0 °C to −15 °C band in stratiform cloud with high liquid water content; freezing rain is the extreme case.
Worked example
You are at FL350 with 1013 set, and the local QNH is 995 hPa. Pressure is 18 hPa below standard, so your true altitude is about 18 × 30 = 540 ft lower than indicated - you are nearer the terrain than the altimeter suggests. The reasoning: a lower QNH means the 1013.2 hPa surface is physically lower over that area, so every flight level sits lower too.
Traps
Confusing QNH and QFE, forgetting the temperature correction on very cold days, treating a TAF as current observation (that is METAR), and reading SPECI as routine rather than as an unscheduled significant change.