The UV Index uses a numerical scale to inform people about the strength of the sun's invisible ultraviolet (UV) rays. The higher the number, the stronger the rays, and the greater the need to take precautions. In Canada, the index ranges from 0 to 10; a value of 11 or higher, while fairly common in tropical areas, is very rare in Canada.
Environment Canada created the original UV Index in 1992 in association with Health Canada and other partners, including the Canadian Dermatology Association, and the Canadian Cancer Society. The purpose of the program is to inform Canadians about the harmful effects of UV radiation – which can cause sunburns, eye cataracts, skin aging, and skin cancer – and to encourage them to take measures to protect themselves.
The Index was adopted by the World Health Organization (WHO) and the World Meteorological Organization (WMO) as an international standard to be used by weather services around the world. It is now in use in more than 25 countries.
In the fall of 2002, the WHO and the WMO published Global Solar UV Index: A Practical Guide to provide guidelines for the use of the Index worldwide. Canada, the United States, New Zealand, Australia, and several European countries have adopted these new guidelines for their UV programs.
Brewer ozone spectrophotometers, the world's most accurate ozone-measuring instruments, on the roof of the Environment Canada building in Toronto. Click to enlarge.
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In addition, research by Environment Canada's Meteorological Service of Canada and others has improved the scientific understanding of stratospheric ozone and UV radiation over the past decade. The renewal of the program was also a chance to incorporate these improvements into the Index.
Specifically, the changes to the Index include:
- Better science: The way Brewer spectrophotometers (a type of ozone-measuring device) are used to determine UV Index values has improved. The forecast Index also now considers the effect of higher elevations and reflection by snow in enhancing UV levels. These changes will result in UV Index forecasts that are only slightly higher for most of the year, but noticeably higher on sunny days with snow on the ground.
- Year-round forecasting: Rather than being included in public forecasts from April to September only, the UV Index is now provided whenever it is expected to reach 3 or above, year-round.
- Refined categories: In the past, UV Index values were grouped as low, moderate, high, or extreme. The thresholds of the categories have changed, and a new "very high" category has been added.
- Round numbers: UV forecasts are now rounded off to the nearest whole number.
- Updated health information: Information on how to protect yourself from UV has been updated for each category of the Index.
The amount of UV radiation a person receives depends on the strength of the sun's rays and the duration of exposure. UV radiation itself varies according to a number of atmospheric, temporal, and geographical factors, including:
- Time of day. UV levels are greatest around midday, when the sun is at its highest point in the sky.
- Season. UV is highest in the spring and summer, lower in the fall and in the winter.
- Cloud cover. The darker the clouds, the less UV that reaches the earth.
- The surface. The brighter the surface you are on, the more UV it reflects back onto your skin. Snow is highly reflective (50-80%), sand and concrete are moderately reflective (12-20%).
- The elevation. You get more UV on a mountain than at lower elevations, because the air is clearer and thinner.
- Where you are. UV is strongest near the equator, and weakens as you approach the poles.
- The ozone layer. The more ozone there is in the stratosphere, the less UVB that reaches the earth. The thinning of the ozone layer over the past few decades has meant greater exposure to the sun's rays.
Listen for the UV Index in your public weather forecast, or find it on Environment Canada's UV Index Web site.
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