The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Visibility and Fog
Aircraft Icing
Weather Reports and Forecasts
Practice Exam

International Standard Atmosphere

The International Standard Atmosphere (ISA) is a set of conditions that are used as a reference point. ISA conditions almost never exist in the real world but we need to be able to compare the real world to the ISA.

ISA Conditions

The International Standard Atmosphere states that:

Sea level temperature is +15°C
Sea level pressure is 1013 hectopascals (hPa)
Sea level density is 1225g/m³

ISA Lapse Rates

The International Standard Atmosphere lapse rates (changes with altitude) are:

Temperature decreases by 2°C per 1000 feet within the Troposphere
Pressure decreases by approximately 1hPa per 30 feet

Comparison to ISA

We compare the real world to ISA in relative terms. For example, the ISA temperature at 2000ft is +11°C (ISA 15°C at sea level minus 2°C per 1000ft). If the current temperature at 2000ft is actually 14°C, we would say it is “ISA +3°C“.