International Standard Atmosphere.

The Airspeed Indicator is calibrated according to assumed conditions of the International Standard Atmosphere (ISA)

In altimetry with the International Standard Atmosphere the hypsometric equation is used to compute pressure at a given height in isothermal layers in the upper and lower stratosphere.

If temperature varies linearly with z (as it is assumed to do in the International Standard Atmosphere), it can also be brought outside the integral when replaced with , the average temperature between and .