Ultraviolet (UV) radiations from the Sun in the spectral range 100-280 nm react with the stratospheric atmosphere, and oxygen molecules (O2) and atoms (O) combine to produce ozone (O3). Since there are destruction processes also, the equilibrium amount is small, only a few percent of the atmosphere. However, it serves a very vital, useful purpose as it absorbs UV in the spectral range 280-320 nm (termed as UVB), which is very dangerous for terrestrial life and is a cause of skin cancer, etc.
