What is the UV Index and how does it work? | Hot Summer Guide | Halifax, Nova Scotia

IIf you’re a subscriber to our newsletter (which you absolutely should be), you’ll have noticed that we provide a weather forecast every morning. Part of this forecast includes today’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 UV can damage your skin, eyes and immune system. The index operates on a linear scale, starting at zero and peaking at 11 or 12 in Canada. There are five levels of risk:

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

Environment Canada recommends that 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 elements: 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, preventing it from reaching us. The thickness of the ozone layer above a given location can change from day to day due to various natural processes. When the sun is high in the sky, UV is strongest because its rays travel directly downward, the shortest path through the atmosphere. When the sun is closer to the horizon, the rays travel farther, so more is absorbed by the ozone layer. Finally, 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, then a computer model predicts the thickness for the next day. Combining this forecast with data on the height and angle of the sun and the amount of cloud predicted by the weather forecast for the next day produces a UV forecast. The index is the greatest amount of UV expected that day, which occurs at noon (or 1:00 p.m. during the day).

Maria D. Ervin