How is ozone produced in the stratosphere?

First, molecular oxygen (consisting of two oxygen atoms joined by a covalent bond) is broken up by ultraviolet radiation. The individual oxygen atoms then combine with other oxygen molecules to form O3.

If you go too high in the atmosphere, there are not enough molecules for the oxygen atoms to form ozone; too low, and there is not enough ultraviolet light to break up significant amounts of molecular oxygen, as reactions in the atmosphere above have absorbed it.