Atmospheric pressure, also known as 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 Pa (1,013.25 hPa), which is equivalent to 1013.25 millibars,[1] 760mm Hg, 29.9212inchesHg, or 14.696psi.[2] 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.
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 metre, 1 N/m2). On average, a column of air with a cross-sectional area of 1 square centimetre (cm2), 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/cm2 or 101kN/m2 (101 kilopascals, kPa). A column of air with a cross-sectional area of 1in2 would have a weight of about 14.7lbf, resulting in a pressure of 14.7lbf/in2 .
Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere.[3][4] It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.[5]
The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level (PMSL). This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they display pressure adjusted to sea level, not the actual local atmospheric pressure.
Average sea-level pressure is 1013.25 hPa (29.921 inHg; 760.00 mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in hectopascals or millibars (1 hectopascal = 1 millibar), except in the United States, Canada, and Japan where it is reported in inches of mercury (to two decimal places). The United States and Canada also report sea-level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars.[6] However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.[7]
In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: 1013.2 hPa (14.695 psi) is transmitted as 132; 1000 hPa (100 kPa) is transmitted as 000; 998.7 hPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1050 hPa (15.2 psi; 31 inHg), with record highs close to 1085 hPa (15.74 psi; 32.0 inHg). The lowest measurable sea-level pressure is found at the centres of tropical cyclones and tornadoes, with a record lowof 870 hPa (12.6 psi; 26 inHg).