Energy from the sun reaches the earth as visible, infrared, and UV rays:
Ultraviolet A (UVA) is made up of wavelengths 320 to 400 nanometers (nm) in length.
Ultraviolet B (UVB) wavelengths are 290 to 320 nm in length.
Ultraviolet C (UVC) wavelengths are below 290 nm in length.
Only UVA and UVB ultraviolet rays reach the earth's surface. The earth's atmosphere absorbs UVC wavelengths:
UVB rays are the main cause of sunburn, and tend to damage the skin's outer layers. These rays play a key role in the development of skin cancer.
UVA rays penetrate the skin more deeply than UVB rays. They play a major part in skin aging and wrinkling, but they also contribute to the development of skin cancers.
In most cases, ultraviolet rays react with a chemical called melanin that is found in the skin. This is the first defense against the sun, as it is the melanin that absorbs the dangerous UV rays that can do serious skin damage. A sunburn develops when the amount of UV damage exceeds the protection that the skin's melanin can provide. While a small amount of exposure to sunlight is healthy and pleasurable, too much can be dangerous. Measures should be taken to prevent overexposure to sunlight in order to reduce the risks of cancers, premature aging of the skin, the development of cataracts, and other harmful effects.
In response to the increasing incidence of skin cancer, cataracts, and other effects from exposure to the sun's harmful rays, the National Weather Service and the EPA collaborated on a sun-awareness information program. An important part of this program is the UV Index.
The Index is a next-day forecast that estimates the amount of ultraviolet radiation that will reach the earth's surface--providing important information to help you prevent overexposure to the sun's rays. The Index also includes the effects of cloud cover on the anticipated UV exposure level for the next day.
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