Question

Why are there few or no clouds in the stratosphere

Answer 1

 Air in the stratosphere is dry and thin
It's the global depression.

Answer 2

In the lower part of the stratosphere, the temperature does not change with height. But, in the upper part , temperature increases with height. At 50km, the temperature is about 7o/c. Air in the stratosphere is dry and thin, and there are very few or no clouds.