Most commercial flights take place in the lowest layer of the atmosphere, called the troposphere. In the troposphere, the air temperature decreases with increasing altitude. The international standard atmosphere (ISA) sets the temperature at mean sea level at 15° C. From this starting point, the ISA assumes that the temperature always drops by 6.5° C per 1,000 m (1.98° C per 1,000 ft) up to an altitude of 11,000 m (36,089 ft).

Above this altitude – the tropopause – the temperature remains constant and then begins to rise again as you go higher. The ISA assumes that the temperature remains constant at -56.5° C for altitudes between 11,000 m and 20,000 m (65,617 ft).

Example 1: The ISA temperature at an altitude of 5,000 m is

15° C – 5 × 6.5°C = -17.5° C

Example 2: The ISA temperature at an altitude of 25,000 ft is

15° C – 25 × 1.98°C = -34.5° C

The actual air temperature encountered by an aircraft at a given altitude is likely to be different from what could be expected from the ISA temperature model. E.g. the static air temperature while flying at an altitude of 33,000 ft may be -39° C whereas the model predicts a temperature of -44° C. In other words, the air at this altitude is 5° C warmer than expected. This difference is usually described as the aircraft being operated in ISA+5 conditions. Tables describing aircraft performance at different altitudes will often use this notation.

Sources:

• getting to grips with aircraft performance, Airbus Customer Services, January 2002
•  ISO 2533:1975, the International Organization for Standardization, Edition 1, 1975