Atmospheric Structure

  
    The atmosphere is divided into several distinct spherical layers, or strata, separated by narrow transition zones.  The study of these layers is called aeronomy.  The temperature structure of the atmosphere of the planet behaves in a manner dependent upon the balance between heating from the sun's incoming radiation, heating from the surface below, and properties inherent to the gases of the atmosphere itself.  Each atmospheric layer is characterized by differences in chemical composition that produce variations in temperature.  The upper boundary at which gases disperse into space extends to several hundred kilometers above sea level.  Being compressible, the air is more dense near the surface of the earth than at higher altitudes. 

    The troposphere is the atmospheric layer closest to the planet and contains the largest percentage of mass of the total atmosphere.  It is characterized by the density of its air and an average vertical temperature change of approximately  6 degrees Celsius per kilometer.  In this layer temperature and water vapor composition decrease rapidly with altitude.  Water vapor is important in regulating air temperature because it absorbs solar energy and thermal radiation from the planet's surface.  The troposphere contains 99% of the water vapor in the Earth's atmosphere.  Water vapor concentrations vary with latitudinal position.  The concentrations are greatest above the tropics and decrease toward the polar regions.  The upper boundary of the troposphere ranges in height from 8 km in high latitudes, to 18 km above the equator.  Its height also varies with seasonal changes; it is highest in the summer and lowest in the winter.  A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere.  Air temperature within the tropopause remains constant with increasing altitude. 

    In general, weather is a tropospheric phenomenon. Troposphere means "region of mixing" and is named because of vigorous convective air currents within the layer.  Clouds frequently are found up to elevations of 9 km, and rarely reach 13 km. The exception is thunderstorms, where they  may get beyond 25 km. The study of meteorology is therefore restricted mainly to a study of a thin boundary layer of the atmosphere about 24 km in thickness. 

    The stratosphere is the second major layer of air in the atmosphere.  It resides between 10 and 50 km above the planet's surface.  The air temperature in the stratosphere remains constant up to an altitude of 25 km.  It then increases gradually to 200-220 degrees Kelvin at the lower boundary of the stratopause, which is marked by a decrease in temperature.  Air temperatures increase with altitude in the stratosphere, which has a stabilizing effect on atmospheric conditions.  Ozone plays the major role in regulating temperature.  Temperatures increase as the ozone concentration increases.  Solar energy is converted to kinetic energy when ozone molecules absorb ultraviolet radiation in heating the stratosphere. 
 
    The mesosphere extends from approximately 50 km to 80 km. It is characterized by decreasing temperatures, which register at about 190-180 K and at an altitude of 80 km.  Because there are decreased concentrations of ozone and water vapor, the temperature is lower than in the troposphere or stratosphere. 

   The thermosphere is located right above the mesosphere, separated by the mesopause.  The temperature in the thermosphere increases with altitudes up to 1000-1500 K.  The increase in temperature is due to the absorption of intense solar radiation by the remaining molecules of oxygen. 

   The exosphere is the most distant atmospheric layer.  It extends to about 960-1000 km.  It is a transitional zone between earth's atmosphere and interplanetary space. 

Atmospheric Layers and Temperature Variations (image) 
Earth's Atmospheric Temperature Structure (image) 

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