Image courtesy University of Illinois.
Atmospheric pressure is the force per unit area at any point in an atmosphere due to the weight of atmospheric gases above it. Atmospheric pressure is measured in millibars, bars, or atmospheres. The standard atmospheric pressure at the surface of the Earth is 1,013 millibars (mb). On Venus the surface pressure is about 90,000 mb (90 bars), while on Mars it is about 7.5 mb.