In the most general terms, the atmosphere is divided into lower and upper regions. The lower atmosphere is generally considered to extend to the top of the stratosphere, an altitude of about 50 kilometers (km). Study of the lower atmosphere is known as meteorology; study of the upper atmosphere is called aeronomy.
The Earth's atmosphere is characterized by variations of temperature and pressure with height. In fact, the variation of the average temperature profile with altitude is the basis for distinguishing the layers of the atmosphere. The regions of the atmosphere are (Figure 1.1):
Troposphere. The lowest layer of the atmosphere, extending from the Earth's surface up to the tropopause, which is at 10-15 km altitude depending on latitude and time of year; characterized by decreasing temperature with height; rapid vertical mixing.
Stratosphere. Extends from the tropopause to the stratopause (From ~ 45 to 55 km altitude); temperature increases with altitude, leading to a layer in which vertical mixing is slow.
Mesosphere. Extends from the stratopause to the mesopause (From ~ 80 to 90 km altitude); temperature decreases with altitude to the mesopause, which is the coldest point in the atmosphere; rapid vertical mixing.
Thermosphere. The region above the mesopause; characterized by high temperatures as a result of absorption of short-wavelength radiation by N2 and 02; rapid vertical mixing. The ionosphere is a region of the upper mesosphere and lower thermosphere where ions are produced by photoionization.
Exosphere. The outermost region of the atmosphere (>500 km altitude) where gas molecules with sufficient energy can escape from the Earth's gravitational attraction.
Over the equator the average height of the tropopause is about 18km; over the poles, about 8 km. By convention of the World Meteorological Organization (WMO), the tropopause is defined as the lowest level at which the rate of decrease of temperature with height (the temperature lapse rate) decreases to 2K km-1 or less and the lapse rate averaged between this level and any level within the next 2 km does not exceed 2K km-1 (Holton et al. 1995). The tropopause is at a maximum height over the tropics, sloping downward moving toward the poles. The name coined by British meteorologist, Sir Napier Shaw, from the Greek word tropos, meaning turning, the troposphere is a region of ceaseless turbulence and mixing. The caldron of all weather, the troposphere contains almost all of the atmosphere's water vapor. Although the troposphere accounts for only a small fraction of the atmosphere's total height, it contains about 80% of its total mass. In the troposphere, the temperature decreases almost linearly with height. For dry air the lapse rate is 9.7 K km-1. The reason for this progressive decline is the increasing distance from the Sun-warmed Earth. At the tropopause, the temperature has fallen to an average of
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