The atmosphere and pressure

Aviation uses the International Standard Atmosphere (ISA): at mean sea level the pressure is 1013.25 hPa and the temperature 15 °C; temperature falls about 2 °C per 1,000 ft up to about 36,000 ft (the tropopause, which is higher — around 50,000 ft — over the tropics).

Air pressure falls with height (about 1 hPa per 27–30 ft near the ground). The altimeter is a barometer marked in feet. Pilots set the local pressure, QNH, so the altimeter shows height above sea level; above the transition altitude everyone uses 1013 hPa and talks in flight levels (FL350 = 35,000 ft on the standard setting).

Hot, high and humid air is less dense. Aircraft need longer runways and climb less well on hot afternoons — important in Malaysia!