The air is thinner and cleaner on a mountaintop - more UV reaches there than at lower elevations. Rainy conditions reduce the amount of UV transmission. Much as clouds shield the Earth's surface from UV radiation, urban smog can reduce the amount of UV radiation reaching the Earth. Incoming UV radiation is reflected from most surfaces. Snow reflects up to 85 per cent, dry sand and concrete can reflect up to 12 per cent.
Water reflects only five per cent.
Reflected UV can damage people, plants, and animals just as direct UV does. Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction. Although natural phenomena can cause temporary ozone loss, chlorine and bromine released from synthetic compounds are now accepted as the main cause of this depletion. Based on data collected since the 1950's, scientists have determined that ozone levels were relatively stable until the late 1970's. Severe depletion over the Antarctic has been occurring since 1979 and a general downturn in global ozone levels has been observed since the early 1980's.