Meteorology: Atmosphere, Weather Hazards and Altimetry
Atmosphere and altimetry
Meteorology questions reward a chain of cause and effect: pressure gradient drives wind, temperature and moisture control stability, stability controls cloud and precipitation, and all of these affect aircraft performance. Know the standard atmosphere (15°C and 29.92 in Hg at sea level, about 2°C per 1,000 ft), the difference between QNH, QFE and standard pressure, and how temperature alters density altitude. Learn the signature weather of stable and unstable air, the formation of radiation, advection and upslope fog, and the hazards of airframe icing, freezing rain, thunderstorms, microbursts and low-level wind shear.
Traps
A frequent trap is using indicated altitude as true altitude in cold air; cold air makes the altimeter over-read, so the aircraft is lower than indicated. METAR wind is reported in true degrees, while runway and ATIS wind may be magnetic-do not mix them. TAF questions often hinge on the valid period, temporary fluctuations and amendment status, not just the first line. Icing questions may ask where the greatest risk lies: stratiform cloud usually gives rime icing, while freezing rain is especially dangerous because of clear ice. Density-altitude questions often give elevation, altimeter setting and temperature; compute pressure altitude first.
Worked example
Aerodrome elevation 1,000 ft, altimeter 29.92 in Hg, temperature 25°C. Pressure altitude equals 1,000 ft. ISA temperature at 1,000 ft is 13°C, so the day is 12°C above ISA. Using the rule of thumb of 120 ft per °C, density altitude is about 1,000 + (12 × 120) = 2,440 ft. That higher apparent altitude increases takeoff distance and reduces climb performance.