troposphere(redirected from Troposhere)
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[Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of
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the part of the atmosphere that is closest to the earth’s surface and contains most of the atmosphere’s mass. Temperature decreases with height in the troposphere, which extends on the average to heights of 8–10 km in polar latitudes, 10–12 km in temperate latitudes, and 16–18 km in tropical latitudes. The stratosphere lies above the troposphere and is separated from it by a relatively thin boundary called the tropopause. More than four-fifths of all atmospheric air is concentrated in the troposphere. The mean atmospheric pressure at the upper boundary of the troposphere in temperate latitudes is less than 25 percent of the pressure at the earth’s surface; in tropical latitudes it is less than 10 percent. All human activity takes place in the troposphere. The highest mountains are within the troposphere, and even the highest-flying aircraft rarely cross from the troposphere into the stratosphere.
The vertical temperature distribution in the troposphere depends on the characteristics of the absorption of solar and terrestrial radiation there and on convective heat transport. The main radiation absorber in the atmosphere is water vapor, the content of which decreases rapidly with height. Consequently, the air temperature must also decrease with height. Such a decrease in temperature promotes the occurrence of convection, which transports heated air upward from the earth’s surface and thus alters the vertical temperature distribution (seeTEMPERATURE DISTRIBUTION IN THE ATMOSPHERE, VERTICAL). As a result, a mean vertical temperature gradient γ, which is equal to 0.6°C/100 m, is established in the troposphere. The value of γ is somewhat lower in the lower troposphere and higher in the upper troposphere. The annual mean air temperature at the upper boundary of the troposphere is about –55°C in polar latitudes and about - 80°C at the equator; in the summer the temperature in the upper troposphere is always considerably below zero. In certain cases the temperature distribution may differ substantially from the average. Temperature inversions—that is, increases in temperature with height—often occur in various tropospheric layers, especially in the lower troposphere.
Most clouds are formed in the troposphere, since it contains virtually all of the atmospheric water vapor. The troposphere also contains the bulk of atmospheric aerosols, such as dust and smoke, which enter it from the earth’s surface. In the lower troposphere, that is, in the atmospheric boundary layer, the diurnal variation of air temperature and humidity is pronounced, the wind speed rapidly increases with height, and the wind direction is nearly the same as that of the lines of equal atmospheric pressure, or isobars. Above the boundary layer the wind speed generally continues to increase, but the wind direction changes in different ways, depending on the tropospheric temperature distribution. The wind speed increases roughly threefold from the boundary layer to the tropopause. In the upper troposphere, very strong jet streams are observed near the tropopause. The part of the lower troposphere that extends to a height of several tens of meters above the earth’s surface is called the lowest atmospheric layer and is the environment inhabited by plants, animals, and man. The wind is especially slack in this layer, but the humidity is high; above the land the vertical temperature gradients, or lapse rates, are very large during the day, but temperature inversions near the surface are common at night.
The system of air currents in the troposphere and lower stratosphere is called the general circulation of the atmosphere. The air in the troposphere divides into air masses whose boundaries are not stationary. The properties of individual air masses differ and depend on latitude and the underlying surface over which the air masses form. Cyclones and anticyclones develop at fronts, which are the boundaries between air masses. The cyclones and anticyclones determine the movement of air masses and fronts and, together with the air masses and fronts, govern aperiodic changes in the weather at the earth’s surface and in the overlying layers. Thus, in addition to the general quasi-zonal transport of air primarily from west to east, air exchange between latitudes occurs in the troposphere. Such air exchange is very important for weather and climate conditions.
REFERENCESKhromov, S. P. Meteorologüa i klimatologiia alia geograficheskikh fakul’tetov, 2nd ed. Leningrad, 1968.
Khrgian, A. Kh. Fizika atmosfery. Leningrad, 1969.
Laikhtman, D. L. Fizika pogranichnogo sloia atmosfery, 2nd ed. Leningrad, 1970.
S. P. KHROMOV
This layer is characterized by a nearly uniform fall of temperature with height. The troposphere contains nearly 75% of the total air by weight in the atmosphere. It is the region where clouds form, convection is active, and mixing is continuous and more or less complete. Most of the weather changes are limited to this layer. See tropopause.