Temperature, Pressure, Density, and Their Altimeter Implications
Core concepts
Understand how temperature, pressure, and density interrelate through the gas law: density falls as pressure falls or temperature rises. Know the pressure altimeter's three settings (QNH, QFE, QNE/standard 1013.25) and what each displays. A pressure altimeter is calibrated to ISA, so it reads true altitude only in ISA conditions; warm air makes it under-read (aircraft higher than indicated), cold air makes it over-read (aircraft lower than indicated). This cold-temperature altimeter error is a recurring exam theme, especially at high-elevation aerodromes.
Density altitude combines pressure altitude and temperature: high, hot, humid conditions raise density altitude, degrading performance. Know that pressure decreases with height, that sea-level pressure analyses use isobars, and that pressure tendency (rising/falling) indicates approaching systems.
Traps
Questions confuse indicated, true, and absolute altitude, or assume QNH and QFE differ by a fixed amount. Another trap: assuming cold air gives a safe margin-it is the opposite. Also watch for confusion between pressure altitude and density altitude.
Worked example
Aerodrome elevation 2000 ft, QNH 1013 hPa, temperature 30 °C. Pressure altitude equals elevation here (2000 ft). ISA temperature at 2000 ft is about 11 °C, so the air is 19 °C warmer than ISA; at roughly 120 ft per degree, density altitude is about 2000 + 2280 ≈ 4280 ft. Plan performance accordingly.