on THE BAsis of thermal characteristics, the atmosphere is normally subdivided into four major vertical layers: the troposphere, stratosphere, mesosphere, and thermosphere. The troposphere makes up the lowest of these layers, extending from the surface to a global average height of 7.5 mi. (12 km.). Coined in 1908 by French scientist Leon Philippe Teisserenc de Bort, the name troposphere is derived from the Greek word tropos, meaning to turn, mix, or change. The term aptly describes the extensive vertical mixing and stability changes of this layer, which generates clouds, precipitation, and other meteorological events. For this reason, the troposphere is commonly referred to as the weather sphere.

The depth of the troposphere is relatively thin, yet it contains approximately 80 percent of the atmosphere's mass. Because the atmosphere is compressible, air molecules are more compact closer to the surface, thereby increasing the density and pressure of the air at lower altitudes. The relationship between density and pressure with altitude is nonlinear, decreasing at a decreasing rate with increasing altitude. In the lower troposphere, the rate of pressure decrease is about 10 mbars. for every 330 ft. (100-m.) increase in elevation.

Temperature in the troposphere generally decreases with height, contrasting considerably between its lower and upper boundaries. Temperature in this layer is largely affected by the radiant energy exchanges from the underlying surface and insolation intensity. The global average temperature at the surface is 59 degrees F (15 degrees C) but decreases to around minus 82 degrees F (minus 63 degrees C) at the top of the troposphere. On the basis of mean tropospheric depth, the average rate of temperature decrease is 3.6 degrees F per 1,000 ft. (6.5 degrees C per km.), a measurement known as the normal lapse rate. This rate represents average global conditions, deviating substantially depending on latitude, time, and local modifications. The actual temperature change with height is the environmental lapse rate, which is measured remotely, using satellites, or directly, using Radiosondes (a balloon-borne instrument package). Eventually, temperature ceases to decline with height, transitioning into a zero lapse rate region (or isothermal layer), where temperature is neither increasing nor decreasing. This shift demarcates the boundary between the troposphere and the stratosphere, known as the tropopause.

The mean height of the tropopause can have considerable spatial and temporal variability. In the tropics, the depth of the troposphere is around 16 km. (10 mi.), but near the poles, the depth dwindles to about 8 km. (5 mi.) or less. The tropopause also varies seasonally, with higher heights occurring during the summer than the winter. Warm surface temperatures occurring at low latitudes and high sun periods encourage vertical thermal mixing, thereby extending the depth of the troposphere. Accordingly, the environmental lapse rate in these regions continues to remain positive (i.e., temperature decreases with height), and tropopause temperatures are typically lower in the tropics than for high latitudes. Occasionally the tropopause is difficult to discern because of extensive mixing between the upper troposphere and the lower stratosphere.

This situation is common in portions of the mid-latitudes, usually defining the location of jet streams (a narrow belt of high-velocity winds often in excess of 185 km. per hour (115 mi. per hour) that steer mid-latitude cyclones. Because the height of the tropopause is dependent on the average temperature of the troposphere, temperature changes in this layer can influence the location of extratropical storm tracks and cloud depth.

Embedded frequently within the troposphere are thin sublayers in which the temperature actually increases with height, known as temperature inversions. Radiation inversions result from nocturnal surface cooling. Under certain ambient conditions (e.g., cloudless nights), terrestrial radiation loss to space is enhanced and the ground (and air above) cool rapidly, thereby establishing a shallow inversion layer. Conversely, subsidence inversions occur from mid-upper tropo-spheric processes that produce areas of sinking air that are being warmed by compression; hence, lower tro-pospheric temperatures are actually colder than those aloft. This setting tends to stabilize the air, inhibiting vertical mixing and cloud growth.

A semipermanent sublayer of the troposphere is the planetary boundary layer (PBL), a section directly influenced by surface daily conditions. Comprising typically the lowest 1 km. (3,300 ft.) of the troposphere, the PBL is characterized by turbulence generated by frictional drag from the surface beneath and rising thermals (heated air parcels). The depth of the PBL amplifies and diminishes with the daily solar cycle, such that the greatest thickness is during the day when the atmosphere is most turbulent.

Evidence suggests that the troposphere has undergone a significant rate of warming during the past century. The tropospheric temperature trend in the latter half of the 20th century is estimated at a 0.18 degree F (0.10 degrees C) increase per decade, similar to the surface temperature rate change. Higher temperatures mean increased surface evaporation and tropospheric water vapor content. As a consequence, cloud cover has also shown an increase, and extratropical precipitation in the Northern Hemisphere has increased 5 to 10 percent since 1900.

Other climate-forcing agents (e.g., anthropogenic-induced greenhouse gas emissions) can alter the Earth's radiation balance and may also explain the upward temperature trend. For instance, tropo-spheric ozone (O3), a greenhouse gas and surface pollutant, has increased by nearly 35 percent since the preindustrial era.

sEE ALso: Atmospheric Boundary Layer; Atmospheric Vertical Structure; Tropopause.

BIBLIoGRAPHY. Intergovernmental Panel on Climate

Change, Climate Change 2001: The Scientific Basis (Cambridge University Press, 2001); Frederick K. Lutgens, Edward J. Tarbuck, and Dennis Tasa, The Atmosphere: An Introduction to Meteorology, 10th ed. (Prentice Hall, 2006); Timothy R. Oke, Boundary Layer Climates, 2nd ed. (Rout-ledge, 1987).

Jill S. M. Coleman Ball State University

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