The standard atmosphere

The International Standard Atmosphere (ISA) is the fixed reference for every altimeter, chart and performance table. Real layers: see atmospheric-layers.

ISA at sea level (TC AIM AIR 1.5.1):

• 1 hPa = 1 mb: renamed millibar. 1013.2 hPa = 1013.2 mb (TC AIM MET 3.3 (1 mb equals 1 hPa); sample exam Q18: 29.92 in. Hg or 1013.2 mb).

Temperature with height (TC AIM AIR 1.5.1(d)):

• Falls 1.98 °C per 1000 ft; rule of thumb about 2 °C (PHAK ch.11).
• ISA temp = 15 - 1.98 x thousands of ft. Example, 10,000 ft: 15 - 19.8 = about -5 °C.
• Falls to the tropopause (see atmospheric-layers), constant above.
• Rate is the surrounding air's, not a rising parcel's (see parcel-temperature-lapse-rates).

Pressure and density both fall with height:

• Pressure falls fastest low down, where most air sits; slower higher up.
• About 1 in.Hg per 1000 ft (TC AIM AIR 1.5.3) up to 10,000 ft (PHAK Principles of Flight, Atmospheric Pressure).
• About half the atmosphere's weight is below 18,000 ft (PHAK ch.12 Atmospheric Pressure).

(PHAK Principles of Flight, Effect of Pressure on Density; Effect of Temperature on Density)

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

Practice