1.10 The International Standard Atmosphere

In order to provide a datum for aircraft performance comparison, and instrument calibration, an assumed set of conditions has been determined. Whilst representative, these conditions do not necessarily reflect actual conditions in the atmosphere. The values used are listed below: 

• Temperature 15 °C at msl, and decreasing at 1.98 °C per 1,000 ft to 36,090 ft (11 km) where the temperature remains constant at -56.5°C until 65,617 ft (20 km).

• Pressure, 1013.25 mb at msl.

• Density, 1.225 kg/cu m at msl.