Troposphere

ON THE BASIS of thermal characteristics, the atmosphere is normally subdivided into four major vertical layers: the troposphere, stratosphere, mésosphère, 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.

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