Understanding the UV Index

[View] (D)

The UV Index is a measure of the intensity of the sun's ultraviolet radiation in the sunburning spectrum. As the UV Index increases, the sun's rays can do more harm to your skin, eyes and immune system and one should therefore take more precautions to protect yourself from the harmful rays.

In February 2004, Environment Canada will change the UV Index program to conform with the new standards set by the World Health Organization. Click here for details.

Environment Canada Scientists developed the UV Index in 1992 as a tool for Canadians to gauge the strengths of the ultraviolet radiation they are exposed to. The UV Index is calculated from data collected at 12 monitoring sites across Canada.The Canadian Meteorological Center uses this data in their weather computer models to issue nation-wide daily forecasts of the next day's UV Index.

The UV Index ranges from 0 to 16+. The highest values are found on mountain tops at the equator. In Canada, the UVI generaly varies from 0 to 10. A UV Index forecast is issued for 48 locations. The UV Index forecast is the maximum value expected for a given day usually around solar noon. Generally, the further south the higher the UV Index. However, the UV Index is also dependent on altitude, reflection, and clouds.

How is the UV Index forecast calculated?

1. The thickness of the ozone layer across North America is forecast using the computer weather prediction models.
2. These values are then corrected based on observations from the 12 ozone-monitoring stations across Canada.
3. This information along with variables on latitude and time of year, is then fed into mathematical algorithms to produce the clear sky UV Index for each desired location.
4. The cloud and precipitation forecasts are generated by meteorologists in each of the regional forecast centres across Canada and are assigned a transmission factor, which is then used to adjust the clear-sky UV forecast.

• Welcome
• Order UV Index Resource Kit
• Resources
• Games & Activities
• UVI Observations
• UVI Maps
• UVI Forecasts
• Science of UV
• Sun Awareness
• Protecting Yourself
• Enter Your Data
• Other Educational Sites

• What's New
• About Us
• Topics
• Publications
• Weather
• Home

The Green Lane^TM,
Environment Canada's World Wide Web Site.