National Physical Laboratory

Why does air density and pressure change with altitude? (FAQ - Mass & Density)

For a given composition and temperature, air density varies in direct proportion to air pressure and this reduces with altitude for two reasons - both related to gravity.

  1. The gravitational attraction (*) between the earth and air molecules is greater for those molecules nearer to earth than those further away - they have more weight - dragging them closer together and increasing the pressure (force per unit area) between them.
  2. Molecules further away from the earth have less weight (because gravitational attraction is less) but they are also 'standing' on the molecules below them, causing compression. Those lower down have to support more molecules above them and are further compressed (pressurised) in the process.

(*) Strictly it is the gravitational force minus the effect of the Earth's spin (an effect that is greatest at the equator).

The graph below gives an indication of how pressure varies non-linearly with altitude. (Apologies for using feet rather than metres but most values of altitude are still expressed in this ancient unit.)

Altitude Vs Pressure
Altitude vs pressure.

Last Updated: 1 Dec 2010
Created: 1 Dec 2010


Please note that the information will not be divulged to third parties, or used without your permission