This places warm air above cooler air, which is a stable arrangement. The result is a layer in which little vertical mixing occurs. That is, the layer is stratified, hence its name, the stratosphere . Normally, neither clouds nor storms reach into this region. The boundary between the troposphere and stratosphere is called the tropopause .
Ozone generating reactions in the stratosphere is the main heat source.
Above the stratosphere, temperature once again falls with increasing elevation, marking the mesosphere . This heating is caused by the action of high-energy radiation from the Sun on the extremely thin air. Their temperature is raised in the process, and because there are so few molecules per cubic centimeter at this height, it takes very little energy to produce large increases in temperature. Because the lower portion of the thermosphere is characterized by the presence of ions, it is also referred to as the ionosphere . The layered structure of the atmosphere is clearly seen in this Space Shuttle image of sun rise.