Temperature, Pressure, Density and Their Altimetry Implications
Temperature and its measurement
Temperature falls with height in the troposphere, but inversions reverse this locally and are common at night over land, in valleys and behind fronts. QNH, QFE and QNE are the three altimeter settings. QNH gives elevation above mean sea level, QFE gives height above the aerodrome, and QNE (1013.25 hPa) gives flight levels.
Pressure
Pressure is the weight of the air above. It decreases roughly 1 hPa per 30 ft near sea level, faster with height in cold air. Isobars show its horizontal pattern; a steep gradient means strong wind.
Density
Density depends on pressure, temperature and humidity. Warm, high, humid air is less dense. Density altitude is pressure altitude corrected for temperature: hot days at a high-elevation airfield can add thousands of feet of effective altitude and badly degrade take-off and climb performance.
Altimetry traps
When flying from high pressure to low pressure without resetting, the altimeter over-reads: the aircraft is lower than indicated. The memory aid is "high to low, look out below". Flying from warm to cold air has the same effect, because the true height of a pressure surface is lower in cold air. Cold-temperature altimeter corrections are required on approach in very cold conditions. Remember that QNH is station pressure reduced to sea level, not the actual station pressure.
Worked example
An airfield at 2000 ft elevation reports QNH 1003 hPa. If the aircraft sets 1013 hPa instead, the altimeter reads about (1013 − 1003) × 30 = 300 ft too high, so the true elevation shown is roughly 2300 ft.