The term surface pressure has two distinct definitions, depending on the context.
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 pressure at mean sea-level (MSL) in the International Standard Atmosphere (ISA) is 1013.25hPa, or 1 atmosphere (Atm), or 29.92 inches of mercury.
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 can also refer to the change of surface tension as a function of the area of water surface available to each molecule in a solution. The measurement of surface pressure is key in determining the monolayer properties of an amphiphilic material.
See also: Pressure: Surface pressure.
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