atmospheric pressure(redirected from Mean Sea Level Pressure)
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atmospheric pressureThe force per unit area (i.e. pressure) exerted by a column of atmosphere extending vertically upwards to the limit of the atmosphere. It is usually expressed in millibars (mb) or newtons per square meter (N m–2). The standard atmospheric pressure at the Earth's surface at sea level is 1013 mb or 101 325 N m–2. On the surface of Venus and Mars the atmospheric pressure is about 90 000 mb and 7.5 mb, respectively.
the hydrostatic pressure exerted by the atmosphere on all objects in it. Atmospheric pressure is a fundamental characteristic of the atmosphere’s condition; at each point in the atmosphere it is determined by the weight of the overlying air. Atmospheric pressure decreases with height; its dependence on height is expressed by the barometric formula. Atmospheric pressure is measured by a barometer and is expressed in millibars (mbar), in newtons per sq m (N/m2), or by the height of a column of mercury in mm in a barometer brought to 0° C and normal gravity value (at sea level and 45° latitude). Normal atmospheric pressure is considered to be 760 mm Hg = 1,013.25 mbar = 101,325 N/m2. At a height of 5 km it is approximately half the value at the earth’s surface.
Atmospheric pressure at the earth’s surface varies from place to place and with time. Of particular importance are the nonperiodic variations associated with the onset, development, and dissolution of slowly moving high-pressure regions called anticyclones and vast, relatively fast-moving vortices, or cyclones, in which low pressure prevails. The extreme values of atmospheric pressure observed so far (at sea level) are 808.7 and 684.0 mm Hg. However, despite the large variability, the distribution of average monthly values at the earth’s surface each year is approximately the same. The average annual atmospheric pressure is low at the equator and has a minimum at about 10° N lat. It then increases and reaches a maximum at 30°-35° north and south latitudes; again it decreases, reaching a minimum at 60°-65°, but at the poles it increases again. This latitudinal distribution of atmospheric pressure is considerably influenced by the time of the year and the nature of the continental and oceanic layout. In winter there is a high-pressure region over the cold continents. Thus, latitudinal distribution is disturbed, and the pressure field is divided into a series of high-and low-pressure regions called centers of atmospheric action. The horizontal distribution of pressure becomes simpler with height, approaching the latitudinal distribution. Starting at a height of approximately 5 km, the atmospheric pressure diminishes from the equator to the poles over the entire globe.
Two maxima are observed in the diurnal variation—at 9–10 A.M. and at 9–10 P.M. Minima occur at 3–4 P.M. and 3–4 P.M. The diurnal variation is especially regular in tropical countries where the daytime variation reaches 2.4 mm Hg and nighttime 1.6 mm Hg. The amplitude of the pressure change decreases with increasing latitude, but at the same time the nonperiodic pressure variations become stronger.
REFERENCESKhrgian, A. Kh. Fizika atmosfery, 2nd ed., ch. V. Moscow, 1958.
Burgess, E. K granitsam prostranstva. Moscow, 1957. (Translated from English.)