The Atmosphere

Atmospheric Structure

If we examine the vertical structure the atmosphere in different places we will find it varies in height, being lowest at the poles and highest at the equator. The varying height is due to the spatial variation in heating of the Earth's surface and thus the atmosphere above. This fact makes it difficult to define exact heights for the layers of the atmosphere. The solution is to subdivide the atmosphere not on the basis of fixed heights, but on temperature change. Figure 3.8 illustrates the way in which the atmosphere is divided using temperature change as the primary criterion. 

Troposphere and Tropopause

The troposphere is the layer closest to the Earth's surface. The graph of temperature change indicates that air temperature decreases with an increase in altitude through this layer. Air temperature normally decreases with height above the surface because the primary source of heating for the air is the Earth. The rate of change in temperature with altitude is called the environmental lapse rate of temperature (ELR) The ELR varies from day-to-day at a place, and from place to place on any given day. The normal lapse rate of temperature is the average value of the ELR, .65^o C /100 meters.  That is, at any particular place and on any given day the actual ELR may be larger or smaller, but on average has a value of .65^o C /100 m. So if I went outside today it could be .62^o C /100 m and then tomorrow it might be .68^o C /100 m. The ELR also varies from place - to - place on a given day. That is, at Chicago, Illinois it might be .65^o C /100 m and on the same day it could be .62^o C /100 m over London, England .

Figure 3.8 Structure of the atmosphere

Under the right conditions, the air temperature may actually increase with an increase in altitude above the Earth. When this occurs we are experiencing an inverted lapse rate of temperature, or simply an inversion. Shallow surface inversions are typical over the snow covered surfaces of subarctic and polar regions, and sometimes occur when high pressure cells inhabit a region. 

The tropopause lies above the troposphere. Here  the temperature tends to stay the same with increasing height. The tropopause acts as a "lid" on the troposphere preventing air from rising upwards into the stratosphere.

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