In an unpolluted atmosphere there is a balance between the amount of ozone being produced and the amount of ozone being destroyed. As a result, the total concentration of ozone in the stratosphere remains relatively constant. Most incoming UV radiation is absorbed by ozone and prevented from reaching the Earth's surface. Without the protective effect of ozone, life on Earth would not have evolved the way it has. Under normal circumstances, the ozone layer is thickest over the poles and thinnest around the equator.
Weather conditions can also cause considerable daily variations.
Ultraviolet radiation is one form of radiant energy coming from the sun. The shorter the wavelength, the more energetic the radiation. Generally, the shorter the wavelength, the more biologically damaging UV radiation can be if it reaches the Earth in sufficient quantity. UV-A, although it is the least energetic form of UV radiation, reaches the Earth in greatest quantity. Most UV-A rays pass right through the ozone layer.