The standard atmosphere

The International Standard Atmosphere (ISA) is the fixed reference every altimeter, chart and performance table is built on. At sea level (TC AIM AIR 1.5.1):

• Temperature 15 °C.
• Pressure 29.92 in.Hg (1013.2 hPa).
• Temperature falls 1.98 °C per 1000 ft as you climb, up to the tropopause near 36,000 ft (that height and the 1013.2 hPa above are from the FAA AWH standard-atmosphere table; the AIM gives only 29.92 in.Hg, 15 °C, the lapse and the -56.5 °C stop), where it levels off at about -56.5 °C and stays there.

Pressure and density both fall with height. Pressure falls fastest in the low levels, where most of the air sits, and slower higher up.

Trap: pressure falls fastest low down, not up high, because that is where the air is densest.

Source note: the ISA reference values (15 °C, 29.92 in.Hg, the 1.98 °C/1000 ft lapse and the -56.5 °C stop) are cited to the TC AIM; the standard-atmosphere table values (tropopause height, 1013.2 hPa) are grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28), a free public-domain text whose physics is universal and matches the Canadian material (equivalent to TP 9352).

Practice