Atmosphere And Weather

How the Uv Index is Calculated

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Exposure to ultraviolet radiation (UV) is a normal part of being outside in the sunshine. A certain amount of UV exposure is important to the production of vitamin D by the human body, but excess exposure can cause health disorders ranging from sunburn to skin cancer and cataracts. According to the World Health Organization, approximately 66,000 deaths occur each year from melanoma and other skin cancers. In an effort to educate the population about UV exposure, authorities have developed a UV index to help people understand the risks and take precautions to limit overexposure.

In the United States, the UV index is produced starting with the measurement of global ozone levels as recorded by two satellites operated by the National Oceanic and Atmospheric Administration (NOAA). A sunlight angle is determined using the latitude, time of day, and the day of the year. Using the sunlight angle and the ozone forecast from the satellites, an estimate of UV strength at ground level is calculated for various points around the country.

According to the Environmental Protection Agency's Sunwise website,  “UV radiation is calculated for several wavelengths between 290 and 400 nm, the full spectrum of UV-B (290-320 nm) and UV-A (320-400 nm) radiation.” The symbol “nm” stands for a nanometer, one billionth of a meter, a measurement unit commonly used to describe wavelengths of light or electromagnetic radiation.

Ozone present in the atmosphere affects shorter wavelengths of UV radiation more readily than longer wavelengths. Additionally, the calculation is weighted for the likelihood of skin damage, as shorter UV wavelengths are more damaging to skin than longer ones. Finally, the strengths of the UV affects at each wavelength are combined to produce a total UV radiation affect on the skin.

UV exposure changes with elevation and is significantly impacted by cloud cover, so the calculation is modified to take these factors into account. The EPA states that “UV intensity increases about 6% per kilometer elevation above sea level,” so UV exposure is a much greater issue on a mountaintop than it is at sea level. Overcast skies can reduce UV penetration by as much as two thirds, compared to the 100% penetration allowed by clear skies.

Once the measurement has been adjusted for all these factors, it is scaled to a whole-number range that begins with 0, representing dark, and extending to 11 or more, representing strong sunlight. Some charts also portray the numbers with a color code, with red indicating a UV index of between 8 and 10, and 11 or higher in purple, warning of a high risk of harm to unprotected skin.

The UV index rating is routinely included in published and broadcast weather forecasts. Recommendations to reduce potential dangers include limiting sun exposure time, wearing hats and other protective clothing, and using sunscreen with broad-spectrum UV protection. UV exposure is cumulative over a lifetime, so repeated excess exposure may lead to serious health issues even if those issues are not immediately apparent.

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