Air pressure is measured by using a barometer, the value of which is given in varying units, including inches of mercury (in Hg) or millimeters of mercury (mm Hg), torr (Tor), bar or millibars (mb), pounds per square inch (psi) and pascal (Pa). Standard atmosphere, which refers to the sea-level atmospheric pressure at 0 degree Celsius, is expressed in atmospheres, commonly abbreviated to atm. One atm is equal to 29.92 in Hg, 760 mm Hg, 760 Tor, 1.013 bar, 1,013.25 mb, 14.7 psi and 101,325 Pa.
Air pressure refers to the force that is exerted by the Earth's atmosphere on a unit surface area. Depending on the geographical location, atmospheric pressures constantly change. Regions of higher elevation have lower air pressures compared to those areas closer to sea level.
There are several types of barometric instruments that measure air pressure. The first mercurial barometer, invented in 1643, consisted of an overturned glass tube containing mercury. As air pressure goes up, the level of mercury rises inside the tube. The pressure is then measured by taking the height of mercury, typically recorded in inches. Instead of using mercury, modern barometers find atmospheric pressure by employing electrical charges. Another type of barometer is called the aneroid barometer. Variations in air pressure are gauged based on whether the sealed wafers inside the device expand or contract. Increases in pressure are indicated when the wafers shrink, while decreases in pressure are determined when the wafers grow in size. Measuring air pressure is an important feature in weather forecasting.