The UV Index is a measure of the strength of the sun’s UV rays. As the UV increases, the sun’s rays can do more harm to your skin, eyes, and immune system.

The UV Index scale

The UV Index scale ranges from a low of zero to a high of 11+ in Canada, The highest values occur in southern Ontario, where values of 11, or even 12 on rare occasions, have occurred.  The sun’s rays become stronger as you move south, and in the tropics, the index can rise to 14 or higher.

The UV index values are grouped into five levels of risk: Low (0-2), Moderate (3-5), High (6-7), Very High (8-10), and Extreme (11+).  The numbers on the scale indicate the strength of the sun’s UV rays.  The higher the number, the stronger the sun, and the greater the need to take precautions.

The UV Index: a Canadian success story

The UV Index was created in1992 by scientists at Environment and Climate Change Canada, as a tool to help protect Canadians from overexposure to the sun.  The scientists set the original UV Index scale from zero to 10, based on the UV values usually found in Canada.

In 1994 the World Health Organization (WHO) and the United Nations Environment Program recognize the value of the UV index, and adopted it as an international program. A global standard, based on the Canadian model, was developed in 2002, and is now used around the world. Since UV values in more southerly countries can be much higher than those in Canada, the WHO and its partners decided to make the scale open ended, so it could rise over 10. UV index values of up to 14 can be found in tropical countries.

How is the UV Index forecast produced?

The UV Index is based on:

• the thickness of the ozone layer in the upper atmosphere: a thick ozone layer absorbs more UV. The ozone layer is now recovering from the long-term effects of industrial pollution, but natural processes can alter its thickness on a daily basis.
• the angle of the sun above the horizon: when the sun is high overhead, its rays travel straight down towards the ground, the shortest route through the atmosphere. This is when UV is the strongest. When the sun is closer to the horizon, its rays travel further through the atmosphere.  Since ozone in the atmosphere absorbs UV, less UV reaches the ground when the sun is lower in the sky.
• the amount of cloud cover: thick clouds can reduce the amount of UV.

To produce the UV Index forecast, Environment and Climate Change Canada takes data from ozone measuring stations across the country to determine the present thickness of the ozone layer. A computer model then forecasts the thickness of the ozone layer over Canada for the next day.

Combining the forecast of the ozone layer with data about the height and the angle of the sun gives a UV forecast for the next day, for a clear sunny sky. Adding the amount of cloud predicted in the weather forecast then gives the amount of UV that can actually be expected.

The forecast gives the maximum amount of UV expected for the day. On a clear day with no clouds, this occurs at noon (or 1 p.m. for those on daylight time), when the sun is highest in the sky.