UV-B damage depends on the amount of atmospheric ozone that can act as a filter, the angle of the sun in the sky, and cloud cover, which shields the surface from some of the ultraviolet radiation. The ozone layer is usually thinnest at the tropics and thickest towards the poles. As stratospheric ozone diminishes, proportionately more of the ultraviolet radiation reaching the Earth s surface will arrive in the shorter UV-B wavelenghts. Fortunately, UV-C is completely absorbed by atmospheric ozone and oxygen. Even with severe ozone reduction, UV-C radiation would still be absorbed by the remaining ozone.
Overcast weather decreases UV-intensity.