Atmospheric Pressure, Altimetry and Pressure Systems
Atmospheric pressure is the weight of the column of air above a surface, measured in hPa (mb), inches of mercury or, in some states, mm Hg. Standard sea-level pressure is 1013.25 hPa. Pressure decreases with height at roughly 1 hPa per 30 ft near sea level, and the rate of decrease slows with altitude.
Altimetry
An altimeter is a barometer calibrated to the ISA. Setting QNH makes it read elevation above mean sea level; QFE makes it read height above the reference datum (usually aerodrome); the standard setting 1013.25 hPa gives pressure altitude and is used above the transition altitude. The transition layer lies between transition altitude and transition level, and the transition level is always at or above the transition altitude.
Altimeter error traps
Fly from high pressure to low pressure without resetting and the aircraft is lower than indicated - the mnemonic is 'high to low, look out below'. Fly from high to low temperature and the true altitude is lower than indicated. The correction is roughly 30 ft per hPa of pressure error and about 4 ft per 1000 ft per °C of ISA temperature error.
Worked example: at 10 000 ft with the air 10 °C colder than ISA, true altitude is about 10 × 4 × 10 = 400 ft below indicated.
Trap: QNH is the pressure reduced to sea level using ISA, not the actual station pressure (that is QFE or QFF).