Meteorology
Atmosphere and altimetry
Meteorology starts with the ISA: sea level 15 degrees Celsius, 1013.25 hPa, lapse rate about 2 degrees per 1,000 ft up to the tropopause near 36,090 ft. Pressure falls roughly 1 hPa per 27 ft near sea level. Altimetry questions hinge on the difference between QNH, QFE and 1013.25, and on temperature error: in cold air the altimeter over-reads and true altitude is lower than indicated, which threatens terrain clearance.
Weather systems
Know stability: stable air gives stratiform cloud and steady precipitation; unstable air gives cumuliform cloud, showers and turbulence. Fronts: warm (gentle slope, wide cloud band), cold (steep, narrow, squall line), occluded. Local effects: sea breeze, anabatic and katabatic flow, Foehn, radiation and advection fog, valley fog. Icing needs visible moisture and temperatures between 0 and minus 20 degrees, worst around minus 2 to minus 10; freezing rain is the most dangerous form.
Reports and forecasts
Read METAR, TAF, SPECI, trend forecasts, SIGMET, AIRMET and GAMET. Decode wind as direction, strength, gusts and variation, and cloud as amount, height and type. Recognise microburst and wind-shear signatures, and know that a thunderstorm cell passes through cumulus, mature and dissipating stages.
Worked example
Temperature at 10,000 ft in ISA = 15 minus (2 × 10) = minus 5 degrees Celsius. A reported outside air temperature of minus 15 degrees means the air is colder than ISA, so the altimeter over-reads and true altitude is lower than indicated.