Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. On a given plane, low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air one square centimeter in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kg and weight of about 10.1 N (A column one square inch in cross-section would have a weight of about 14.7 lbs, or about 65.4 N).
Standard Atmospheric Pressure
The standard atmosphere (symbol: atm) is a unit of pressure equal to 101325 Pa or 1013.25 millibars. It is equivalent to 760 mmHg (torr) or 14.696 psi.
Mean Sea Level Pressure
The mean sea level pressure (MSLP) is the atmospheric pressure at sea level. 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 measure pressure reduced to sea level, not the actual local atmospheric pressure. The reduction to sea level means that the normal range of fluctuations in atmospheric pressure is the same for everyone. The pressures that are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools. The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.
QNH
The barometric altimeter setting that will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield.
QFE
The barometric altimeter setting that will cause an altimeter to read zero when at the reference datum of a particular airfield (in general, a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.
QFE and QNH are arbitrary Q codes rather than abbreviations, but the mnemonics “Nautical Height” (for QNH) and “Field Elevation” (for QFE) are often used by pilots to distinguish them. Average sea-level pressure is 101.325 kPa (1013.25 mbar) or 760 millimetres of mercury (mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars, except in the United States, Canada, and Colombia where it is reported in inches (to two decimal places) of mercury. However, in Canada’s public weather reports, sea level pressure is reported in kilopascals. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1050.0 mbar (105.00 kPa), with record highs close to 1085.0 mbar (108.50 kPa). The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 870 mbar (87 kPa).
Atmospheric Pressure Records
The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1,085.7 hectopascals (32.06 inHg) measured in Tosontsengel, Mongolia on December 19, 2001. The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata, Evenhiyskiy, Russia on December 31, 1968, of 1,083.3 hectopascals (31.99 inHg). The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (25.69 inHg), set on October 12, 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft. The normal high barometric pressure at the Dead Sea (lowest terrestrial point below sea level), as measured by a standard mercury manometer and blood gas analyzer, was found to be 799 mmHg (1065 hPa).
Boiling Point of Water
Clean fresh water boils at about 100 °C (212 °F) at earth’s standard atmospheric pressure. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water. Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. This is why cooking at elevations more than 1,100 m (3,600 ft) above sea level requires adjustments to recipes. A rough approximation of elevation can be obtained by measuring the temperature at which water boils.