UV Index: What is it?

The UV Index is a forecast of the amount of skin damaging UV radiation expected to reach the earth's surface at the time when the sun is highest in the sky (solar noon). The amount of UV radiation reaching the surface is primarily related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds present. The UV Index can range from 0 (when it is night time) to 15 or 16 (in the tropics at high elevations under clear skies). UV radiation is greatest when the sun is highest in the sky and rapidly decreases as the sun approaches the horizon. The higher the UV Index, the greater the dose rate of skin damaging (and eye damaging) UV radiation. Consequently, the higher the UV Index, the smaller the time it takes before skin damage occurs.

There are two prices to pay for overexposure to UV radiation: a severe sun burn following an intense short term overexposure, and the more serious skin cancers developing after long term overexposure. Long term overexposure to UV radiation has been linked to the formation of cataracts in the eyes as well.

Using U.S. forecasts of UV

The U.S. NWS forecasts of UV levels use these numbers:

NumberExposure level
0 1 2 Low
3 4Minimal
5 6Moderate
7 8 9High
10 or higherVery high

The table below shows what the levels imply.. "Time to burn" and "actions" apply to people with a Type II, fair skin that sometimes tans and usually burns. People with lighter skin need to be more cautious. People with darker skin may be able to tolerate more exposure. But even dark skin can burn.

LevelTime to burnActions to take at noon
Minimal60 minutesApply SPF sunscreen
Low45 minutesApply SPF sunscreen, wear a hat.
Moderate30 minutesApply SPF 15, wear a hat
High15-24 minutesApply SPF 15 to 30 sunscreen, wear a hat and sunglasses. Limit midday exposure
Very High10 minutesApply SPF 30+ sunscreen. Wear a hat, Sunglasses, and protective clothing
How to avoid sunburn

The best protection against ultraviolet radiation is to stay out of the sun. But no one wants to stay indoors all the time. The best time to enjoy the sun, especially in tropical locations or places near the tropics, is before 10 a.m. and after 3 p.m. local time.

Anyone who lives outside the world's tropical or semi-tropical regions should be especially careful when traveling to the tropics. The sun is always higher in the sky there than in places farther north or south.

Clouds do not screen out all UV radiation. Also, sand, snow and water reflect UV radiation, which means being in the shade under a beach umbrella does not provide complete protection. Wearing a hat with a wide brim is a good idea to protect the eyes, ears and neck. Most clothing, except white, reflects or absorbs the radiation, but wet cotton, no matter what color, can let a large amount through.

Sunglasses that block UV rays are vital to protect the eyes from the sun. Eye doctors can test eyewear for ultraviolet protection.

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