UV Index Calculator

This page lets you find out approximately what UV Index you can expect anywhere on earth and at any time of year.

The UV Index varies according to several factors. So the numbers you get will be only approximations.

• The angle that the sun is. This depends on where you are, the time of year and the time of day. When it is vertical, the rays take the shortest path through the ozone layer, so the UV Index is the highest.
• The thickness of the ozone layer. This depends on where you are and the time of year. It also changes from day to day. It can vary naturally by up to 25 percent from normal. In the polar ozone holes it can decrease by as much as 70 percent.
• Your altitude above sea level. The UV Index increases by 4 percent per 300 metres (1000 feet) of altitude.
• Other factors. If there is snow on the ground it will increase. In a snowy city, about 10 percent. In a snowy forest, about 25 percent. In the tundra or a treeless plain, about 40 percent. If you are in the mountains you will get an increase due to altitude and also an increase due to snow.
• Thick clouds will reduce the UV Index. Thin clouds will reduce it only a little, or not at all. If you can see your shadow, the Index is not reduced.

This is a graph for March 1. It shows how the UV Index during the day. Solar time is approximately local standard time. If you are on Daylight Saving (Summer) time, 12 o'clock solar time will be about 1 p.m.

This is a graph showing the highest UV Index you can expect for every day of the year. Click on a part of it to get a graph for a new date, showing how the UV Index varies during the day.