What is the UV Index?

Darlene Goodman

Many scientists suggest that the sun’s ultraviolet (UV) light waves are harmful to human skin and eye tissue. As a result, the World Health Organization (WHO) created the UV index, a standard system for measuring the amount of UV light that penetrates Earth’s atmosphere. The linear scale is one way for governments and scientists to measure and track ultraviolet light intensity, as well as to warn the public about potential dangers associated with high UV levels.

Many sunglasses are labeled with a UV rating.
Many sunglasses are labeled with a UV rating.

UV radiation is a specific set of wavelengths on the light spectrum. The waves are shorter than visible light. Often classified as UVA and UVB, these light waves are typically considered more dangerous to the skin and eyes than visible light. The UV index measures the amount of these potentially harmful waves that reaches the lower atmosphere.

The sun produces UV radiation.
The sun produces UV radiation.

There are several factors affecting the ultraviolet light in a given area. First, the position of the sun in the sky is often important. Seasons can affect UV by changing the sun’s distance and angle of light in relation to Earth. Also, UV is often strongest at latitudes closer to the equator.

For the most part, the UV index measures the intensity of light waves at solar noon, or the time of day when the sun is highest in the sky. Sunlight is typically strongest at this time. Solar noon may not be the same as noon on a clock.

Second, atmospheric conditions may affect ultraviolet levels, as well. A thicker atmosphere results in lower radiation, so the UV index level is often different from mountain to valley. Cloud cover may also have an effect, but it does not make a large change, because UV radiation can typically penetrate clouds better than other light wavelengths. In addition, ozone in the high atmosphere may filter harmful UV rays.

Finally, ground reflection may also play a role in the UV index. Snow, water, and sand can reflect UV light. This reflection can intensify the level of ultraviolet rays striking an individual outdoors in these conditions because, not only do they receive UV directly from the sun, but it is reflected back up at them from the ground.

There are several ways to limit an individual’s exposure to UV light. Sunglasses with UVA and UVB filters can protect a person’s eyes from damaging rays. Sunscreens of at least Sun Protection Factor (SPF) 15 are often recommended to protect skin.

Individuals may also wish to seek shade, remain indoors, or at least avoid direct sunlight during peak sunlight hours. Depending on the time zone, this period usually falls between 10:00 a.m. and 4:00 p.m. People may also wish to cover their skin by wearing long sleeves, trousers, and a wide-brimmed hat.

Ozone protects the Earth from deadly ultraviolet radiation.
Ozone protects the Earth from deadly ultraviolet radiation.

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Discussion Comments


Besides providing a UV index forecast, a local weather service -- in conjunction with the EPA -- can also issue warnings and advisories if the atmospheric conditions make going outside for extended periods of time potentially harmful.

A UV Alert will be issued if the next day's UV forecast is unusually high for a given area in a given time of year. Essentially it's an opportunity to remind people about sun safety (using sunscreen, covering skin exposed to the sun's rays, wearing sunglasses, etc.) when the risk of sunburn and other damage is especially high.

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