Time of day. Daily changes in the angle of the sun influence the amount of UV radiation that passes through the atmosphere. When the sun is low in the sky, its rays must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components. The greatest amount of UV reaches the Earth around midday when the sun is at its highest point. 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.