ATMOSPHERIC PRESSURE

also called Barometric Pressure, force per unit area exerted by an atmospheric column (that is, the entire body of air above the specified area). Atmospheric pressure usually is measured with a mercury barometer (hence the commonly used synonym barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere the base of which coincides with that of the mercury column. Also, it may be measured using an aneroid barometer, in which the sensing element is one or more hollow, partially evacuated corrugated-metal disks supported against collapse by an inside or outside spring; the change in the shape of the disk with changing pressure can be recorded using a pen arm and a clock-driven revolving drum. Atmospheric pressure is expressed in several different systems of units: inches (or millimetres) of mercury, pounds per square inch (psi), dynes per square centimetre, millibars (mb), atmospheres, or kilopascals. Standard sea-level pressure, by definition, equals 29.92 inches (760 mm) of mercury, 14.70 pounds per square inch, 1013.25 103 dynes per square cm, 1013.25 millibars, one atmosphere, or 101.35 kilopascals. Variations about these values are quite small; for example, the highest and lowest sea-level pressures ever recorded are 32.01 inches (in the middle of Siberia) and 25.90 inches (in a typhoon in the South Pacific). The small variations in pressure that do exist largely determine the wind and storm patterns of the Earth. Near the Earth's surface the pressure decreases with height at a rate of about 3.5 mb for every 30 m (100 feet). The pressure at 270,000 m (10-6 mb) is comparable to that in the best man-made vacuum attainable. At heights above 1,500 to 3,000 m (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken.