Too much exposure to the sun's ultraviolet rays can cause sunburn, skin cancer, skin aging and cataracts.

What is the UV Index and how does it work?

Don't get burned: The ozone layer tells us when to reach for the sunscreen

If you’re subscribed to our newsletter (which you definitely should be), you’ll have noticed we provide a weather forecast every morning. Part of that forecast includes the day’s UV Index—but what is the UV Index anyway?

As defined by Environment Canada, the UV Index measures the strength of the sun’s UV (ultraviolet) rays. The stronger the rays and the more time you spend in the sun, the more damage UV can cause to your skin, eyes and immune system. The index works on a linear scale, starting at zero and peaking at 11 or 12 in Canada. There are five risk levels:

  • Low (0-2)
  • Moderate (3-5)
  • High (6-7)
  • Very High (8-10)
  • Extreme (11+)

Environment Canada recommends you protect your skin and eyes from the sun when the UV Index is 3 or higher by wearing sunscreen, sunglasses and a hat.

Fun fact: The UV Index was developed by scientists in Canada in 1992, and has since been standardized and adopted internationally.

But how is it measured? The index is based on three things: The thickness of the ozone layer, the angle of the sun above the horizon and the amount of clouds in the sky. The thicker the ozone layer, the more UV it absorbs, keeping it from getting to us. The thickness of the ozone layer above a given location can change from day to day due to a variety of natural processes. When the sun is high in the sky, UV is strongest because its rays travel straight down—the shortest route through the atmosphere. When the sun is closer to the horizon, the rays travel further, therefore more gets absorbed by the ozone layer. Lastly, the thicker the clouds, the more UV they absorb.

To calculate a UV forecast, scientists take data from ozone measuring stations across the country to find out the layer’s thickness, and then a computer model forecasts the thickness for the next day. Combining that forecast with data about the height and angle of the sun and the amount of cloud predicted by weather forecasts for the next day produces a UV forecast. The index number is the highest amount of UV expected on that day, which happens at noon (or 1pm during daylight time.)

About The Author

Kaija Jussinoja

Kaija Jussinoja is a news reporter at The Coast, where she covers the stories that make Halifax the weird and wonderful place we call home. She is originally from North Vancouver, BC and graduated from the University of King’s College in 2022. Jussinoja joined The Coast in May 2022 after interning at The Chronicle...

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