Why do temperatures increase in the stratosphere?
Ozone creation and destruction.
The reason the atmosphere gets colder in the troposphere the further you go up is because the earths surface is heated by the sun. Thus, the further you get from the surface the colder it gets. The opposite is true in the stratosphere. The stratosphere gets progressively warmer the higher you go because the ozone layer is located in the stratosphere. When sunlight breaks apart molecules and they recombine into ozone in the ozone layer, heat is a byproduct of that reaction. And because of that reaction/ where it takes place, the heating of the stratosphere is opposite the troposphere. This is also the reason why the stratosphere is a relatively stable region of the atmosphere with virtually no vertical mixing.