Force per unit area exerted by the air above the surface of the Earth. Standard sea-level pressure, by definition, equals 1 atmosphere (atm), or 29.92 in. (760 mm) of mercury, 14.70 lbs per square in., or 101.35 kilopascals, but pressure varies with elevation and temperature. It is usually measured with a mercury barometer (hence the term barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere above it. It may also be measured using an aneroid barometer, in which the action of atmospheric pressure in bending a metallic surface is made to move a pointer.
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The pascal (symbol: Pa) is the SI derived unit of pressure, stress, Young's modulus and tensile strength. It is a measure of perpendicular force per unit area i.e. equivalent to one newton per square meter or one joule per cubic metre. In everyday life, the pascal is perhaps best known from meteorological barometric pressure reports, where it occurs in the form of hectopascals (1 hPa = 100 Pa).. In other contexts, the kilopascal is more commonly used, for example on bicycle tire labels. One hectopascal corresponds to about 0.1% and one kilopascal to about 1% of atmospheric pressure (near sea level): one hectopascal is thus equivalent to a millibar; one atmosphere is equal to 1013.25 hPa.
The unit is named after Blaise Pascal, the eminent French mathematician, physicist, and philosopher noted for his experiments with a barometer, an instrument to measure air pressure. The name pascal was adopted for the SI unit newton per square metre by the 14th CGPM in 1971.
Standard atmospheric pressure is 101,325 Pa = 101.325 kPa = 1013.25 hPa = 1013.25 mbar = 760 Torr. This definition is used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.
In 1985, IUPAC recommended that standard atmospheric pressure should be harmonized to 100,000 Pa = 1 bar = 750 Torr. The same definition is used in the compressor and the pneumatic tool industries (ISO 2787).
The Unicode computer character set has dedicated symbols ㎩ for Pa and ㎪ for kPa, but these exist merely for backward-compatibility with some older ideographic character-sets and are therefore deprecated.
Another unit for pressure measurement in common use today is millimetres of water (1 mmH2O = 9.80665 Pa).
Meteorologists worldwide have for a long time measured atmospheric pressure in millibars. After the introduction of SI units, many preferred to preserve the customary pressure figures. Therefore, meteorologists use hectopascals (hPa) today for air pressure, which are equivalent to millibars, while similar pressures are given in kilopascals in practically all other fields, where the hecto prefix is hardly ever used. Since official metrication, meteorologists in Canada use kilopascals (kPa), see for example CTV News, weather; current conditions in Montréal and CBC weather, current conditions in Montréal, although in some other countries hectopascals are still in use, see for example KNMI, KMI, DWD, JMA, MDD and NOAA
Vehicle owners' guides now specify tire inflation in kilopascals.
Airtightness testing of buildings is measured at 50 Pa or 0.2 inches of water.