As the sun grows stronger and the days grow longer in the spring, Environment Canada's weather reports change. Instead of wind-chill factor, you're likely to hear about the UV index, a gauge of the amount of ultraviolet light (UV) reaching the ground.

Ultraviolet light is part of sunlight, but it's outside the range of light that our eyes can see. Nevertheless, it can do us damage. Exposure to ultraviolet light, or UV radiation, is the main cause of skin cancer. UV radiation can also damage eyesight by contributing to the development of cataracts.

Ultraviolet light has a shorter wavelength than visible light and a higher frequency or energy level, explains Michael Purves, a meteorologist with Environment Canada. Sunburn is your skin's painful reaction to that energy. The less obvious effects of the energy are possible damaging changes to cells in your skin or eyes.

"There are three kinds of UV, differentiated by wavelength," says Purves. "UV-A has the shortest wavelength, followed by UV-B and UV-C. The latter two forms are the ones that reach ground level."

Most of the UV-A disappears in the upper atmosphere, where it is absorbed by reactions with oxygen molecules, made up of two atoms of oxygen. When the high-energy UV-A light hits the molecules, it breaks apart the two atoms. Usually they recombine, says Purves, but sometimes an oxygen atom will attach itself to an oxygen molecule to form ozone, which is made up of three oxygen atoms.

"The ozone absorbs UV at different wavelengths than UV-A," says Purves. "It's those wavelengths that are very damaging to us."

Ozone is constantly forming and being destroyed in the upper atmosphere, but the balance of ozone creation and destruction remained relatively even until recently.

"The problem for us is that the ozone is being destroyed by manufactured chemicals," says Purves. "As ozone is destroyed in the upper atmosphere, more harmful UV is getting down to the surface of the earth."

The most alarming symptom of ozone depletion was the discovery several years ago of what has been called a hole in the ozone layer over Antarctica. Since then, a similar but smaller "hole" has been detected over parts of the Arctic, well east of the Yukon.

"It's not really a true hole," Purves explains. "It's not that there isn't any ozone, just that there's not very much."

The thinning of the ozone layer occurs over the poles, the meteorologist says, because the reactions that destroy ozone are more efficient when it's very cold. Temperatures in the upper atmosphere are coldest over Antarctica, where they can drop to -80 degrees Celsius in the Antarctic spring.

Although the ozone layer is thinnest near the poles, the dangers of UV radiation are highest at the middle latitudes where the sun is directly overhead. In northern latitudes where the sun is low in the sky, Purves says, its light meets the atmosphere at an angle and therefore travels longer through atmosphere before it reaches the ground. The more atmosphere the light passes through, the more UV radiation is absorbed.

In the southern Yukon, the highest UV exposure comes in late spring when days are long, the sun is relatively high, and there's still plenty of snow and water to reflect the light. However, the UV index in the Yukon still remains in the moderate range (about 5). In southern Canada, it can shoot up to the high or even extreme range (7 and upwards) in spring and summer.

Although the UV danger is relatively small in the Yukon, it's still wise to take precautions like sunscreen and sunglasses, especially at higher altitudes where more UV penetrates, says Purves.

"Altitude makes a big difference, because you're getting higher in the atmosphere," he explains.

For more information about ultraviolet radiation or ozone depletion, see the Yukon State of the Environment Report or contact Environment Canada in Whitehorse.