Effects of Clouds, Elevation, and Surface Pollution?
Clouds, air pollution, haze and elevation
all have affects on the amount of ultraviolet (UV) radiation reaching the surface. UV
radiation reaches the surface as a sum of its direct component (normal to the sun) and its
diffuse component (from all directions). UV radiation reaches the top of the troposphere
in mostly its direct component. This is because there are few molecules to scatter the
radiation. Decreases in UV radiation intensity has resulted due to absorption by ozone.
Once the UV radiation reaches the troposphere it encounters much greater numbers of
scattering air molecules and dust. Below are how UV radiation is affected by:
Elevation: In the troposphere, air molecules and dust increase as the UV
radiation travels from the stratosphere to the troposphere. The further down in to the
atmosphere UV radiation travels, the more the direct component is reduced and the more the
diffuse component is increased. As more UV radiation is scattered, the smaller the amount
that reaches the surface. As a result, there is more UV radiation at higher elevations
than at lower elevations.
Clouds: Made up of millions of water droplets, clouds can transmit, reflect
and scatter UV radiation. The amount of each is dependant upon the thickness of the cloud
and its morphology. Generally, the larger and thicker the cloud is the lesser amount of UV
radiation that is transmitted. UV radiation can and does reflect off the sides of towering
cumulus clouds. Such conditions result in actual enhancements of surface UV radiation.
Dust/Haze: These two conditions act on UV radiation the same way. They both
scatter UV radiation. Enough UV radiation is scattered that on hazy or dusty days there is
less UV radiation reaching the surface than would otherwise be there on a clear day.
Air Pollution/Smog: This encompasses many greenhouse gases. Emissions from
traffic and manufacturing plants form smog as UV radiation and heat cause the necessary
chemical reactions to take place. As a result, the amounts of UV radiation reaching the
surface is smaller under these conditions.