The stratosphere begins at the tropopause, which forms its lower boundary. The lower stratosphere is isothermal in character. In other words, the temperature in the lower part of this sphere does not change with altitude.
In certain situations, there may be slight increase in temperature with elevation. This temperature region is found to be present up to about 30 kilometers. There is also persistence of its circulation patterns and high wind speeds.
The circulation changes, if they occur at all, are very rapid. Cirrus clouds, called the mother-of- pearl clouds, occasionally form in the lower stratosphere. Above the tropopause no visible weather phenomena ever occur.
There is a gradual temperature increase with height beyond 20 kilometers. This region is known as the upper stratosphere.
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In summer, the increase in the stratospheric temperature with latitudes continues upto the poles. But during the winter season the stratosphere is warmest between latitudes 50°-60°.
From latitude 60° pole-ward the temperature decreases again. The thickness of the stratosphere is highest at the poles. The upper boundary of the stratosphere is called the stratopause. Above this level there is a steep rise in temperature.