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	Minimal	30	>120	hat
3	Low	15	90	hat, sunscreen (SPF 15 or more)
4		15	75	hat, sunscreen (SPF 15 or more)
5	Moderate	12	60	hat, sunscreen (SPF 15 or more), shady areas
6		10	50	hat, sunscreen (SPF 15 or more), shady areas
7	High	8.5	40	hat, sunscreen (SPF 15 or more), stay indoors from 10 am to 4 pm
8		7.5	35	hat, sunscreen (SPF 15 or more), stay indoors from 10 am to 4 pm
9		7	33	hat, sunscreen (SPF 15 or more), stay indoors from 10 am to 4 pm
10	Very high	6	30	stay indoors as much as possible, take other precautions when outdoors
11		5.5	27
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%.