UV Index: Information

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.

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