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ultraviolet index

A scale indicating the predicted intensity of ultraviolet radiation at noon standard time (1 p.m. daylight savings time), provided in weather forecasts as a way of warning susceptible individuals of the danger of sunburn on particular days. In Canada, the first nation to provide such predictions, the scale runs from 0 to 10. On June 24, 1994, the National Weather Service added a UV index to its weather forecasts for 58 U.S. cities. The U.S. scale runs from 0 to 15 in order to accommodate the higher radiation levels found in the more southerly country. Australia officially inaugurated UV forecasts in 1996.

Index value	Exposure category	Minutes to burn for people who never tan	Minutes to burn for people who rarely burn	Precautions (U.S. E.P.A. recommendations)
0-2	Low	30	>120	Wear sunglasses on bright days. If you burn easily, cover up and use broad spectrum SPF 30+ sunscreen. Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.
3	Moderate	15	90	Stay in shade near midday when the sun is strongest. If outdoors, wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating. Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.
4		15	75
5		12	60
6	High	10	50	Reduce time in the sun between 10 a.m. and 4 p.m. If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating. Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.
7	High	8.5	40
8	Very high	7.5	35	Minimize sun exposure between 10 a.m. and 4 p.m. If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating. Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.
9		7	33
10		6	30
11	Extreme	5.5	27	Try to avoid sun exposure between 10 a.m. and 4 p.m. If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses. Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating. Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.
12		5	25
13		<5	23
14		4	21
15		<4	20

In predicting UV levels, meteorologists take into account such factors as the angle of the sunlight (determined by latitude and the time of year), altitude (about 1 index unit more for each 4000 feet in summer) and cloud cover.

A person's immediate environment can also greatly effect exposure to UV. While grass absorbs about 97% of the UV falling on it, sand reflects 20 to 30%, snow and ice 80 to 90%, and the surfaces of bodies of water can reflect 100%.

For today's UV index for the United States, and forecasts, enter your zipcode at:

https://www.epa.gov/sunsafety/uv-index-1

Unfortunately, websites that purport to provide the uv index anywhere in the world so far have limited coverage. If you know of any reliable sources for individual countries, please email us their URL.

further resources

John P. Kinney, Craig S. Long and Alan C. Geller.
The Ultraviolet Index: A Useful Tool.
Dermatology Online Journal, vol. 6, no. 1, (2000).
Available online at https://escholarship.org/uc/item/5925w4hq.

Craig S. Long, Alvin J. Miller, Hai-Tien Lee, Jeannette D. Wild, Richard C. Przywarty and Drusilia Hufford.
Ultraviolet Index Forecasts Issued by the National Weather Service.
Bulletin of the American Meteorological Society, vol. 77, no. 4 (April 1996)
doi: 10.1175/1520-0477(1996)077<0729:UIFIBT>2.0.CO;2

Reviews of UV index apps for smartphones and tablets:

https://dailyburn.com/life/tech/sun-safety-apps-for-uv-index/