• The vertical structure of the atmosphere can be characterized many ways, three of which are:
• Chemical composition
• Electrical charge
• Temperature
• division according to temperature variation with altitude
• troposphere - temperature decreases with altitude
• stratosphere - temperature increases with altitude
• mesosphere - temperature decreases with altitude
• thermosphere - temperature increases with altitude
• a pause region between each "sphere"
• "heating source" for each region
• Earth's surface is main heat source for the troposphere (absorption of IR by trace gases & clouds, and convection/conduction)
• Sun heats the stratosphere (absorption of UV by ozone)
• Atmosphere and Earth below heat mesosphere (absorption of IR by CO2)
• Sun heats the thermosphere (absorption of X-rays & UV by oxygen)

At the very bottom of the 5 A.M. sounding (below 900 mb) the temperature actually increases with altitude.  This is NOT the stratosphere.  We must go through the troposphere before we get to the stratosphere.  This often referred to as the boundary layer.  Here the temperature is most influenced by the transfer of heat via conduction and convection of energy from the surface.  Above this region (about 900 mb), the transfer of heat is influenced more by IR radiation from both the surface and the atmosphere itself.  So, even though the temperature actually increases with altitude right next to the surface, this is still in the troposphere.