The ICAO Standard Atmosphere or ISA is a standard against which to compare the actual atmosphere at any point and time. (ICAO = International Civil Aviation Organization).
The real atmosphere differs from ISA in many ways. Sea level pressure varies from day to day and there are wide extremes of temperature at all levels. Variation in pressure, vertically and horizontally, affects the operation of the pressure altimeter.
At mean sea level, the ISA parameters are:
- Pressure = 1013.25 hPa
- Temperature = +15 °C
Each of these parameters decreases when the height from mean sea level reference increases:
The ISA model divides the atmosphere into layers with linear temperature distributions.
|Layer||Layer name||Altitude above MSL (in km)||Lapse Rate (°C/km)||Temperature (in °C)||Atmospheric Pressure (hPa)|