The Standard Atmosphere

The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth’s atmosphere change over a wide range of altitudes or elevations. It has been established to provide a common reference for the design and testing of aircraft along with instrument calibration. The ISA models a hypothetical standard day to allow a reproducible engineering reference for calculation and testing of engine and vehicle performance at various altitudes.

The following points must be committed to memory!

The Standard Sea Level Temperature is 15°C
The Standard Sea Level Pressure is 29.92 in. Hg
The Standard Lapse Rate is 1.98°C / 1,000 feet. This is often simplified to 2°C/1,000 feet

The ISA also models the height of the tropopause but does not represent its true height which varies with latitude and time of year. In the ISA the height of the tropopause is approximately 36,000 feet and the assumed temperature is -56.5°C.

ISA Deviation

Meteorological observations are made in absolute figures. However, in some cases, when making calculations regarding aircraft performance temperatures relative to ISA must be considered. These are known as ISA deviations. For example, if the observed temperature is 18°C at sea level (3°C warmer than that expected in the ISA) the deviation would be +3°C.