Theory
In the standard atmosphere, pressure decreases with height along a polytropic profile: in the troposphere the temperature falls linearly by 6.5 K per km and the air is in hydrostatic equilibrium, which gives equation (1). At 1000 m the ambient pressure is about 89 900 Pa, at 2000 m about 79 500 Pa. The temperature profile of the standard atmosphere enters only through equation (1); the gas temperature of the case is Temperature.
The altitude changes the absolute reference pressure of the case, and with it the gas density (lower at altitude). The same mass flow therefore gives a higher velocity and a larger pressure drop.
Equations
| where | meaning | unit |
|---|---|---|
| Ambient pressure | Pa | |
| Altitude (this setting) | m |
Default and rule
- Default
- 0 m
- Rule
0 m (101 325 Pa).
- Source
The troposphere of the standard atmosphere (U.S. Standard Atmosphere, 1976); the exponent is (see Ambient pressure: mode).
Valid range -500 to 11 000 m
Values
-500 to 11 000 m. Used only in altitude mode; in pressure mode Atmospheric pressure takes precedence.
Top of the troposphere
Equation (1) holds up to 11 km only; higher values are rejected.