UV Index Reporting
Using Weather Forecast Models to Predict Ultraviolet Radiation Level
Mar 10, 2008
Ultraviolet Radiation
Ultraviolet radiation is emitted by the sun, as part of electromagnetic spectrum. UV rays from the sun are either:
- UVC rays - absorbed by the atmosphere
- UVB rays – about 10 percent reach the Earth’s surface
- UVA rays – most reach the Earth’s surface
Worldwide UV Index Reporting
The National Weather Service (NWS) and the United States Environmental Protection Agency (EPA) developed the UV Index in 1994, to provide daily forecasts for expected risk of sun overexposure. In May 2004, the World Health Organization along with the World Meteorological Organization developed the Global UV Project, initiating international guidelines for UVI reporting. The new UV index scale has been used since May 2004.
The UV Index in the United States
The UV Index is a scale used to predict daily levels of ultraviolet radiation reaching the Earth’s surface. Values are given on a scale from 1 to 11+, 1 being the lowest and 11+ the highest.
- Reading of 2 or less means low danger
- 3-5 UV Index reading means moderate risk of harm
- Reading of 6-7 means high risk of harm if unprotected from sun
- 8-10 reading on the UV Index means very high risk of harm
- 11+ means extreme risk of harm, take all precautions to avoid sun exposure on these days
The UV Index predicts values for each day at noon, as the sun reaches its highest point. Every zip code in the United States is given a UV Index value, taking into account local weather conditions and forecast models from the NWS.
Factors Affecting UV Radiation
There are several factors that can affect the intensity of UV radiation reaching the Earth’s surface, according to the EPA's A Guide to the UV Index:
- Time of Day – UV radiation peaks at noon or midday, with lower levels in the early morning and late afternoon
- Cloud Cover – Mostly cloudy skies can block most of the UV radiation from reaching the Earth’s surface. Thin or patchy clouds let most UV radiation through. Fair weather clouds can deflect rays and increase the amount of radiation hitting the Earth’s surface.
- Ozone – UV radiation is absorbed by ozone. Higher amounts of ozone mean fewer UV rays reach the surface of Earth
- Altitude – UV radiation increases with altitude because air is thinner. The EPA averages a 2 percent increase in UV radiation for every 1,000-foot increase in altitude.
- Seasons – UV radiation peaks in the spring and summer, with the lowest levels in winter.
- Surface of the Earth – Some surfaces like snow, sand and water, can reflect UV rays. Structures and trees can reduce exposure to UV radiation.
- Latitude – The strongest UV rays occur at the equator, lessening the closer to the poles.
UV Alert System
When UV radiation is predicted to be high, at a value of 6 or higher, the EPA will provide an alert for the region. The UV Alert will be issued along with the UV Index forecast for the area of concern, available online or from the local weather station.
Sun Overexposure Health Risks
Prolonged UV radiation exposure can result in negative health effects, such as sunburn and long term damages such as premature aging, cataracts and even skin cancer. It is important to note that small amounts of UV radiation absorption are beneficial to health, as the production of vitamin D occurs naturally from the sun.
For further reading about the benefits of vitamin D from the sun, make sure to read:
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