About the UV Index

The Ultraviolet (UV) Index is a measure of the intensity of the sun's ultraviolet radiation in the sunburn spectrum. As the UV Index increases, the sun's rays can do more harm to your skin, eyes and immune system. This is why it is important that you listen for Environment Canada's daily UV Index in your local weather forecast. It will help you gauge what precautions are necessary to protect you and your family from harmful UV rays.

Environment Canada scientists developed the UV Index in 1992 as a health protection tool for Canadians to gauge the strength of the ultraviolet radiation they are exposed to. It conforms with the standards set by the World Health Organization (WHO) and is forecast for 48 locations across Canada.

The UV Index ranges from 0 to 11+ (the higher the number, the higher the UV levels). The highest values are found on mountain tops at the equator. In Canada, the UV Index generally varies from 0 to 10.

The UV Index forecast is the maximum value expected for a given day at solar noon. Generally, the further south in latitude, the higher the UV Index. However the UV Index is also dependent on altitude, reflection, clouds and of course, stratospheric ozone concentrations.

Environment Canada has developed a set of sun protection messages for each category of the UV Index. It is divided into five levels: Low (0-2), Moderate (3-5),  High (6-7), Very High (8-10), and lastly, Extreme (11+). These categories refer to the strength of the sun's UV rays. The stronger the sun's rays are, the greater the need to take precautions.

Click here to access Environment Canada's recommended sun protection actions for each level of the UV Index.

How is the UV Index forecast calculated?

  1. The thickness of the ozone layer across Canada is forecast using computer weather prediction models.
  2. These ozone values are then corrected based on observations from the nine 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 for atmospheric cloud conditions, which is then used to adjust the clear-sky UV forecast to obtain the actual daily UV forecast value.