troposphere

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troposphere The troposphere is the part of the atmosphere in which all ‘weather’ occurs: clouds, rain, snow, hurricanes, tornadoes, or fronts. The name comes from the Greek tropos, ‘turning’. In this part of the atmosphere turbulent overturning of air is dominant and causes mixing vertically as well as the horizontally.

The atmosphere is divided into layers or ‘spheres’ according to the different properties found in each layer. If the layers are characterized by the rate at which temperature changes with height, then the atmosphere is split into four layers: the troposphere, stratosphere, mesosphere, and thermosphere. If the layers indicate the chemical composition, they are referred to as the homosphere and the heterosphere, according to whether the composition is homogeneous or heterogeneous. Below about 80km altitude the atmosphere is homogeneous: it has the same chemical composition at all locations. Other layers given special names are the ionosphere, where some of the atoms and molecules are electrically charged, and the exosphere, where some molecules escape from the gravitational attraction of the Earth and move off into space.

The troposphere is a layer in which, on average, the temperature decreases with height. It extends from the Earth's surface to the tropopause, the name given to the boundary between the troposphere and the stratosphere (Fig. 1). In the stratosphere the temperature is constant at lower levels and begins to rise at greater altitudes. The tropopause can be defined as the place where the temperature stops decreasing with height and becomes constant. The altitude of the tropopause is not constant. It varies from place to place and from time to time. It is usually higher in the tropics, where it is about 16 km, than near the poles, where it is about 6 to 8 km. At middle latitudes, between about 40 and 60° N and S, the tropopause lies between about 10 to 12 km. Its height varies from day to day, depending on the large-scale movement of air within the troposphere.

The average rate at which the temperature decreases with height in the troposphere is 6.5 degrees Celsius in every kilometre. Since this is a decrease in temperature it is usually referred to as the lapse rate. If we were to measure the lapse rate on any particular day, at an arbitrary location, it would probably not be 6.5 degrees Celsius per kilometre. We would find that the observed lapse rate would almost never exceed 10 degrees Celsius per kilometre but would sometimes be very small. In fact, the lapse rate can often be zero, in what is known as an isothermal layer (since the temperature in the layer is constant). The lapse rate can also be negative, indicating that the temperature is increasing with height. This is called an inversion layer, since it is an inversion of the usual decrease in temperature with height. Inversion layers and isothermal layers are usually not very deep: rarely more than a few hundred metres.

Because air is constantly overturned in the troposphere, it is worth considering the changes that are forced on the air as it moves. In order to imagine these changes consider a ‘parcel’ of air: a mass of air which will move about without mixing with its surroundings as if it were enclosed in an invisible, totally flexible balloon. If this air parcel lies near the Earth's surface it will gain or lose heat according to whether it is colder or warmer than the surface. The surface itself gains heat during the day by absorbing sunlight and loses heat at night when there is a net heat loss through infrared radiation to space.

The air parcel is most likely to be warmed during the day. (Some air parcels will be warmed more than others, because some parts of the surface will be warmed more than others.) As the air parcel is warmed, it expands: the greater the heating, the greater the expansion. Because the air in the parcel now occupies more space, it has a lower density than the relatively cooler air parcels. The less dense air parcels will rise. This rising air is part of a thermal, a rising current of air which develops over parts of the surface that are warmer than their surroundings.

Because the atmospheric pressure decreases with height, the rising air parcel encounters air around it which has a lower pressure. As the parcel rises, the pressure inside it will tend to become the same as the pressure outside. This causes the air parcel to expand further. Expansion requires work to be done against the surrounding air. At the surface the energy for the expansion came from the heat provided by the warm surface. In the atmosphere there is no such source of energy. The energy needed for expansion has to come from the heat within the air parcel. As the air parcel rises it thus cools as it expands. This can be explained in physical terms by the first law of thermodynamics, through which we can calculate the rate at which the air parcel cools. The cooling rate is 10 degrees Celsius for every 1 km of ascent. This is referred to as the dry adiabatic lapse rate: ‘dry’ because no water has condensed in the air parcel; ‘adiabatic’ because all the energy for expansion comes from the air parcel itself; ‘lapse rate’ because the air parcel cools as it rises.

Because the rising air is cooling at a rate greater than the commonly observed lapse rate, the air parcel will eventually cool until it has the same temperature as the surrounding air. It will also have the same pressure and density. It will then no longer have any buoyancy, and it will stop rising; see Fig. 2.

The rising air will contain some water vapour and will often be unsaturated as it begins to rise. As it cools it will eventually reach a temperature at which it becomes saturated. Further rising will cause condensation of water into drops. This is the lifting condensation level, which marks the base of clouds formed in the rising air. As the air rises further, more water condenses. As the water condenses it releases latent heat. The rate at which the air cools as it rises is less after condensation starts, since the latent heat that is released warms the air slightly. The amount of warming depends on how much water vapour condenses. The lapse rate under these conditions, the saturated adiabatic lapse rate, is less than the dry adiabatic lapse rate. It varies from place to place and from time to time, but is about 6.5 degrees Celsius per kilometre on average—the same as the average lapse rate observed in the troposphere.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994). Meteorology today. West Publishing Co.
McIlveen, J. F. R. (1986) Basic meteorology. Van Nostrand Reinhold, New York.

troposphere The layer of the atmosphere between the Earth's surface and the tropopause, within which the air temperature on average decreases with height at a rate of about 6.5°C/km, though variations that sometimes occur include inversions (temperature increase with height within some limited layer). Most of the atmospheric turbulence and weather features occur in this layer, which contains almost all the atmospheric water vapour and most of the aerosols in suspension in the atmosphere (although there is also an important aerosol layer at about 22 km). See also ATMOSPHERIC STRUCTURE.

troposphere The layer of the atmosphere between the Earth's surface and the tropopause, within which the air temperature on average decreases with height at a rate of about 6.5°C/km, though variations that sometimes occur include inversions (temperature increase with height within some limited layer). Most of the atmospheric turbulence and weather features occur in this layer, which contains almost all the atmospheric water vapour and most of the aerosols in suspension in the atmosphere (although there is also an important aerosol layer at about 22 km).

trop·o·sphere / ˈträpəˌsfi(ə)r; ˈtrō-/ • n. the lowest region of the atmosphere, extending from the earth's surface to a height of about 6–10 km (the lower boundary of the stratosphere). DERIVATIVES: trop·o·spher·ic / ˌträpəˈsfi(ə)rik; -ˈsferik; ˌtrō-/ adj.

troposphere The lowest distinct layer within the atmosphere, extending from sea-level to the tropopause at an altitude of 10–15 km. Weather systems are confined principally to the troposphere, which accounts for about 75 % of the Earth's total atmospheric mass. Temperatures fall with increasing height in the troposphere, reaching a minimum at the tropopause.

troposphere The lowest level of the earth's atmosphere, extending from the earth's surface to a height of about 10 km (its thickness varies from 7 km at the poles to 28 km at the equator). Within the troposphere temperature falls with increasing height, although temperature inversions can occur.

troposphere see atmosphere .