Atmospheric pressure

Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101.325 kPa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, 760torr (or about 760mmHg), about 29.9212inHg, or about 14.696psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately one atm.

In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation. Because the atmosphere is thin relative to the Earth's radius – especially the dense atmospheric layer at low altitudes – the Earth's gravitational acceleration as a function of altitude can be approximated as constant and contributes little to this fall-off. Pressure measures force per unit area, with SI units of pascals (1 pascal = 1 newton per square meter, 1N/m). On average, a column of air with a cross-sectional area of 1 square centimeter (cm), measured from the mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or "weight" of about 10.1 newtons, resulting in a pressure of 10.1 N/cm or 101kN/m (101 kilopascals, kPa). A column of air with a cross-sectional area of 1in would have a weight of about 14.7lbf, resulting in a pressure of 14.7lbf/in.