Surface pressure often refers to the atmospheric pressure at a location on Earth's surface. It is directly proportional to the mass of air over that location.
For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional logarithm of surface pressure.
The average value of surface pressure on Earth is 985hPa. This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea-level for locations above or below sea-level. The average mean sea-level pressure is 1013.25hPa, or 1 atmosphere (Atm).
Pressure (P), mass (m), and the acceleration due to gravity (g), are related by P = F/A = (m*g)/A, where A is surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.
Surface Pressure - Area Isotherms
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