Pressure measures force per unit area, with SI units of pascals (1 pascal = 1 newton per square metre, 1 N/m 2). 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. As elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. 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 1 atm. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa 1,013.25 mbar), which is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. For the pressure of air in other systems, see Pressure.Ītmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.
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