How this calculator works
Hydrostatic balance and the ideal-gas law relate pressure to geopotential height. In the isothermal model temperature is constant through the entire column. In the lapse model the entered temperature is the sea-level value and temperature decreases by 6.5 K/km. Both assume dry air and constant standard gravity. The lapse option ends at 11 km; the uniform-temperature option is an approximation allowed through 20 km. Weather and humidity can make real pressure profiles differ.
Formula
Isothermal: p = p0 exp[-g h/(R T)]. Lapse model: p = p0 [1-Lh/T0]^(g/(R L)), L = 0.0065 K/m, R = 287.05287 J/(kg K), g = 9.80665 m/s2.
Worked example
At zero altitude pressure equals the sea-level reference pressure. With constant T = 288.15 K and p0 = 101325 Pa, pressure at 1000 m is about 89996.67 Pa.
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Frequently asked questions
Is the entered pressure absolute or gauge?
All pressure values here are absolute. Zero gauge pressure does not mean zero atmospheric pressure.
What does temperature mean in each model?
It is the uniform column temperature for isothermal air, and the sea-level temperature for the lapse model.
Why stop the lapse model at 11 km?
The standard lower stratosphere has a different temperature profile. Extending a tropospheric lapse indefinitely is not physical.