Meteorology
Meteorology tests your ability to turn weather information into a go/no-go decision and to anticipate how temperature, pressure, wind, and moisture change aircraft performance. Expect questions on altimetry, density altitude, stability and cloud type, fronts, icing, thunderstorms, fog, and wind shear.
Altimetry and density altitude
An altimeter set to QNH reads elevation at the field; set to 29.92 inHg (1013.25 hPa) it reads pressure altitude. Cold air or lower pressure makes the altimeter over-read: the aircraft is lower than indicated ('high to low, look out below'). Density altitude is pressure altitude corrected for temperature; high temperature, high elevation, and low pressure all raise it and degrade takeoff, climb, and landing performance.
Stability, cloud, and fronts
Steep lapse rates mean unstable air: cumuliform cloud, showers, turbulence, and good visibility between showers. Stable air gives stratiform cloud, smooth air, haze, drizzle, and fog. A cold front arrives fast with a wind shift, squall line, thunderstorms, and a rapid temperature drop; a warm front gives gradually lowering cloud, drizzle, and icing risk. Carburetor icing is most likely at 0 to +20°C with high humidity; structural icing needs visible moisture and near-freezing temperatures.
Thunderstorms, fog, and wind
Avoid thunderstorms and their gust fronts, hail, and microbursts; do not attempt to fly beneath a storm. Radiation fog forms on clear, calm nights with a small temperature-dew point spread; advection fog forms when warm moist air moves over a cold surface. Surface wind is weaker and more gusty than gradient wind, and mechanical turbulence increases near hills and buildings.
Worked example
Field elevation 2,000 ft, altimeter 29.92, temperature 30°C. Pressure altitude = 2,000 ft. ISA temperature at 2,000 ft = 15 − 2 × 2 = 11°C. The deviation is +19°C. Using the rule of thumb of 120 ft per °C, density altitude ≈ 2,000 + (19 × 120) = 4,280 ft. That is a 2,280-ft penalty: expect a longer takeoff roll and a weaker climb, so recompute performance rather than using sea-level figures.
Traps that cost marks
Treating field elevation as density altitude on a hot day; forgetting the cold-air altimeter error; assuming stable air means no icing; using surface wind as the wind at 2,000 ft; and believing a distant storm is harmless.