Pressure at any point on the Earth's surface that is due to the weight of the column of air above it; it therefore decreases as altitude increases, because there is less air above. Particles in the air exert a force (pressure) against surfaces; when large numbers of particles press against a surface, the overall effect is known as air pressure. At sea level the average pressure is 101 kilopascals (1,013 millibars, or 760 mm Hg, or 14.7 lb per sq in, or 1 atmosphere). Changes in atmospheric pressure, measured with a barometer, are used in weather forecasting. Areas of relatively high pressure are called anticyclones; areas of low pressure are called depressions.
For every square metre of a surface, a force of 10 tonnes of air pressure is exerted. This air pressure does not crush objects because they exert an equal amount of force to balance the air pressure. At higher altitudes the air is thinner and the air pressure is lower. Here, water boils at a temperature less than that at sea level. In space there are no air particles and so no air pressure is exerted on an astronaut's body. Astronauts wear spacesuits that supply an air pressure against their bodies.
A barometer is an instrument used to measure air pressure. If a glass tube is filled with mercury and turned upside down with its end in a bowl of mercury, then the height of the column of mercury is held by the air particles pressing on the mercury in the bowl. This measures the air pressure. A standard measurement for atmospheric pressure at sea level is a column of mercury 760 mm/30 in high. At higher altitudes the height of the mercury column would be less, as the air pressure is lower.
Depressions and anticyclones
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