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UV Index EPA Calculator

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Updated 8.12.11
This script calculates through the EPA server, the daily UV index, though not available for Alaska and Hawaii. The Ultraviolet (UV) Index, developed in 1994 by the National Weather Service (NWS) and the US Environmental Protection Agency (EPA), helps Americans plan outdoor activities to avoid overexposure to UV radiation and thereby lower their risk of adverse health effects. EPA and NWS report the Index as a prediction of the UV intensity at noon, though the actual UV level rises and falls as the day progresses. The ozone layer shields the Earth from harmful UV radiation. Ozone depletion, as well as seasonal and weather variations, cause different amounts of UV radiation to reach the Earth at any given time. Developed by the National Weather Service (NWS) and EPA, the UV Index predicts the next day's ultraviolet radiation levels on a 1-11+ scale, helping people determine appropriate sun-protective behaviors. Guidelines for reporting the UV Index have been revised according to guidance from the World Health Organization.

Previously the UV Index was reported on a scale of 0 to 10+, with 0 representing "Minimal" and 10+ representing "Very High." The new global scale (see below) now uses a scale of 1 (representing "Low") to 11 and higher (representing "Extreme"), a new color scheme, revised exposure categories, and different breakpoints between exposure categories. A UV Index of "0" is still possible, but there is no corresponding health message because there either is no UV at that level or the amount is trivially small.

The sun emits three types of ultraviolet radiation; UVA, UVB and UVC. UVA penetrates deep into the skin often causing intense damage like wrinkles and skin discoloration. Exposure to UVB causes sunburn, a skin reaction where blood vessels expand and leak fluids, producing inflammation, pain and redness. Sunburn, whether severe or mild, can cause permanent and irreversible skin damage. Cumulative exposure to UV radiation and the number of severe sunburns received, especially during childhood, significantly increases the risk of developing skin cancer.

The ozone layer blocks the sun's output of UVC and most UVB radiation. The UVB radiation that does reach the earth's surface poses the greatest danger for sunburn and skin damage. Ozone gas high in the atmosphere is vital in filtering out much of the sun's UV, making ozone depletion a major environmental issue. Decreasing ozone levels will increase the health risks associated with UV, but personal risk can be largely averted by simply avoiding the sun during the middle of the day, or by covering up with appropriate clothing, headgear and sunglasses, and by using SPF 30+ sunscreen.

Forecasting the intensity of UV at ground level takes into account information factors on the time of day, date, latitude, amount of cloud, altitude, presence of haze and ozone concentrations. The EPA developed a global computer model that predicts ozone concentration, and calculates the intensity of UV radiation expected to reach the ground at any point. The forecast radiation intensity is converted to an index that estimates the maximum UV intensity for midday, assuming clear skies. Forecasters then apply a correction factor for the anticipated cloud coverage.

Cloudy and cool days always deceive many people into thinking the danger of UV radiation is minimal or is not there at all. Clouds affect the strength of radiation reaching the ground in different and very complex ways. Most clouds block at least some UV radiation, but the degree of protection depends on the type and amount of cloud cover. Some clouds can have the opposite effect and actually increase the UV intensity on the ground by reflecting and refracting the sun's rays. People can also be caught off guard when a small break in an overcast sky allows a brief burst of intense radiation to reach the ground. Cold air can also be deceptive as temperature if often thought of as having a direct bearing on UV and sunburn, however it is not directly related to UV intensity. Skiers should take particular care as reflective snow on the ground and high altitude raise the UV Index significantly relative to its value at sea level.

The UV Index is a simple and informative way of describing the daily danger of solar UV radiation intensity. UV highest intensity remains close to the midday peak value between 10am and 2pm, (11am and 3pm during daylight saving time). Without adequate protection, sun exposure between these hours poses the greatest risk for developing severe sunburn rapidly. You can still burn outside of these hours; however it will take somewhat longer. Each point on the index scale is equivalent to 25 milliwatts/square meter of UV radiation at the earth's surface for UV wavelengths between 290 and 400 nanometers.

The calculation and results are supplied on the EPA server. Use your browser's BACK button to return to our site.
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