ICAO Standard Atmosphere (ISA)
The ICAO Standard Atmosphere (International Standard Atmosphere, ISA) is a mathematical model of the atmosphere. The ICAO standard atmosphere (ISA) has been introduced in order to provide an equal system of altitude reference for the movements of all aircraft in the atmosphere.
Source: ICAO Doc 7488 Manual of the ICAO Standard Atmosphere (3rd ed., 1993, unchanged); ISO 2533.
ISA standard values at sea level (MSL)
| Parameter | ISA value |
|---|---|
| Atmospheric pressure | 1013.25 hPa (= 1013.2 hPa in many PPL texts, = 29.92 inHg) |
| Air temperature | +15 °C |
| Relative humidity | 0 % ("0 % has been defined as relative humidity in the ICAO standard atmosphere") |
| Air density | 1.225 kg/m³ |
| Speed of sound | 340 m/s |
Temperature lapse rate
The assumed temperature decrease with altitude in the ICAO standard atmosphere is 0.65 °C / 100 m:
- Applies up to the tropopause (ISA height 11 000 m = 36 090 ft).
- Equivalent to ~2 °C / 1000 ft (cockpit rule of thumb).
- Above the tropopause: constant temperature -56.5 °C in the stratosphere to ~20 km.
Key ISA values with altitude
| Altitude AMSL | Pressure | Temperature |
|---|---|---|
| 0 ft / 0 m | 1013.25 hPa | +15 °C |
| 5000 ft / 1524 m | 843 hPa | +5 °C |
| 18 000 ft / 5500 m | ~ 506 hPa ("at 5500 m AMSL, the atmospheric pressure is equal to half the pressure at sea level") | -21 °C |
| 36 090 ft / 11 000 m (tropopause) | 226 hPa | -56.5 °C |
→ At ~5500 m AMSL the atmospheric pressure equals half the MSL pressure — an important reference.
Pressure altitude
Pressure altitude is defined as altitude above the 1013.2 hPa level:
- It is the vertical distance above the standard datum plane where atmospheric pressure is 1013.2 hPa — represents the altitude in the ISA at that pressure level ("pressure altitude is the vertical distance above the standard datum plane where atmospheric pressure is 1013.2 hPa, representing the height in the International Standard Atmosphere at that pressure level").
- When the altimeter is set to standard pressure, it indicates aircraft pressure altitude.
Altimeter and ISA
A barometric altimeter in aircraft operates by correlating measured air pressure to corresponding altitudes based on the International Standard Atmosphere model, providing an indicated altitude that assumes standard conditions:
- The altimeter is calibrated to standard atmosphere — deviations of the actual atmosphere from ISA cause incorrect altitude indications relative to true altitude.
Terms — altitude vs height vs elevation
- Altitude is the vertical distance above MSL.
- Elevation is the vertical distance of a location above MSL — i.e. altitude for a fixed surface point (airfield elevation, mountain elevation).
- Height = vertical distance above ground (AGL). Height becomes zero when the aircraft touches the ground.
Main application
- For flight-level operation (above transition altitude): all aircraft use 1013.25 hPa → common ISA reference → vertical separation ensured.
- Performance calculations in the AFM rely on ISA + corrections (DA factors).