The International Standard Atmosphere: An Overview

What is the International Standard Atmosphere?

The International Standard Atmosphere (ISA) is a static atmospheric model that describes the pressure, temperature, density, and viscosity of the Earth's atmosphere. It provides a standardized reference for aeronautical and meteorological applications, and is widely used for aircraft performance calculations and weather forecasting.

ISA Model

The ISA model is based on data collected from atmospheric sounding balloons, aircraft, and satellites. It assumes a standard sea level pressure of 1013.25 hPa (hectopascals), a sea level temperature of 15°C (59°F), and a lapse rate of 6.5°C per kilometer (19.8°F per mile) up to an altitude of 11,000 meters (36,000 feet).

Beyond 11,000 meters, the ISA model assumes a constant temperature of -56.5°C (-69.7°F) up to an altitude of 20,000 meters (65,000 feet), and then a constant temperature of -131°C (-203.8°F) above 20,000 meters (65,000 feet).

The ISA model is not an exact representation of the actual atmosphere, but provides a useful average for most aeronautical and meteorological purposes.