# Atmospheric Pressure Formulae

The ground level atmospheric pressure on a planet is not fixed by any of the standard planetary data such as gravity and temperature. It depends on just how much atmospheric gas is available and has not yet escaped into space. For example, Venus is a little smaller than Earth and has slightly lower graity, yet its atmospheric pressure is about 90 times that of Earth. You are pretty free to set the ground level atmospheric pressure to whatever you like.

Once the ground level pressure is decided on, the atmospheric pressure P at other altitudes is governed by the following formula:

P = Po exp(-g h / H)
where
Po = the ground level pressure
g = the surface gravity in units of Earth gravities
h = the altitude
H = the atmospheric scale height
On Earth the scale height is 7400 metres. On other worlds it will be
H = Ho Mo / M
where
Ho = the scale height on Earth (7400 metres)
Mo = the mean molecular weight of Earth's atmosphere (29 atomic mass units)
M = the mean molecular weight of the planet's atmosphere
For breathable nitrogen-oxygen atmospheres, M will be close to 29.

The following JavaScript form calculates the atmospheric pressure at any altitude on an alien world.

 Surface pressure (atmospheres): Surface gravity (Earth gravities): Atmospheric mean molecular weight (atomic mass units): Altitude (metres): The pressure at this altitude is: