Atmospheric pressure is the
pressure at any point in the
Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the
hydrostatic pressure caused by the
weight of
air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air 1 square inch in cross section, measured from
sea level to the top of the atmosphere, would weigh approximately 14.7
lbf. A column of air would weigh about 100
kilonewtons (equivalent to a mass of 10.2
tonnes at the surface).
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A high, 'high-pressure, or high pressure area is a region where the
atmospheric pressure is greater than the surrounding area. In some countries, these regions may be referred to as
anticyclones. Highs are frequently associated with light winds and
subsidence. Subsidence will generally evaporate most cloud droplets after less than 500 meters, due to
adiabatic heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight or reflected infrared, there is more incoming shortwave
solar radiation and temperatures are higher in the summer and lower in the winter. At night, the absence of clouds means that outgoing
longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler
diurnal low temperatures in all seasons.
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