The answer is: A little higher than it has been in the past, but not unusually high, thanks to this year's polar vortex.
The level of risk posed by the sun in summer depends on the condition of the ozone layer that protects us. Located in the stratosphere 15 to 35 kilometres above the ground, the ozone layer acts like a screen that filters out many of the sun's damaging ultraviolet rays. The thicker it is, the more protection it offers.
What you may not have known is that its thickness depends on the polar vortex that we heard so much about this winter. The Arctic phenomenon early this year unleashed extreme cold snaps and heaped snow on southern Canada, freezing the Great Lakes until they were covered in the most ice in 20 years. But because of all that, there's no hole in the ozone layer this year, scientists report.
Small increase in UV index
Unfortunately, the ozone layer is still on the thin side compared with what it used to be, thanks to ozone-depleting chemicals that were pumped into the atmosphere before they were banned by an international agreement called the Montreal Protocol in 1989.
Overall, Environment Canada expects summer ozone levels in the stratosphere to be one to two per cent below the levels they were before the 1980s, the department told CBC News in an email. That corresponds to a one to two per cent increase in the UV Index — the scale that measures your risk of sun "overexposure" and the amount of sun protection you need — under clear sky conditions.
But there is also some variation across the country. In May, ozone levels were at pre-1980s levels over Edmonton, but about nine per cent below those levels over Resolute, Nunavut. Environment Canada said that was likely the result of natural variability.
Regardless, being cautious about your exposure to the sun is a good idea. A recent report from the Canadian Cancer Society found more Canadians are getting melanoma, the deadliest form of skin cancer, which is linked to damage from the sun's ultraviolet rays. The society was particularly concerned because skin cancer is highly preventable — all you need to do is protect yourself from the sun.
That said, summer levels of ultraviolet radiation are generally higher than they were in the past. While most major ozone depleting chemicals have been banned for some time, many of them are long-lasting and remain in the atmosphere for decades.
Ozone layer recovery?
"The ozone layer, of course, has been in decline for some years," said Thomas Duck, an atmospheric scientist at Dalhousie University in Halifax. "But the sense of the scientific community is that it's pretty much bottomed out, we think.… We hope that it's going to start to recover, but there's not any solid evidence for that yet."
In the meantime, the ozone layer over Canada has good and bad years, thanks to the now notorious polar vortex.
The Arctic polar vortex is a stable patch of isolated air that circles above the North Pole (there is an Antarctic one over the South Pole too) in the stratosphere.
In 2011, the Arctic polar vortex was unusually cold and stable, likely because of warmer temperatures near the surface. That led to the formation of polar stratospheric clouds made of ice, where ozone-depleting chemicals accumulate. When the sun shone down and warmed things up in the spring, it released the chemicals en masse and kicked off a chain of ozone-depleting chemical reactions.
This happens almost every spring in the Antarctic, producing an annual ozone hole in the Southern Hemisphere, but it doesn't normally happen over the Arctic. That's because winds coming over the Rocky Mountains usually end up disturbing the polar vortex at some point each winter, said Felicia Kolonjari, a PhD candidate in atmospheric physics at the University of Toronto.
But the vortex wasn't disturbed in 2011, leading to the first ever ozone hole generated over the North Pole. That, in turn, resulted in higher exposure to ultraviolet rays for Canadians that summer as the hole drifted south.
Scientists at the PEARL research laboratory in Eureka, Nunavut, take measurements to monitor the polar vortex and the ozone layer each year.
This year, they report, the polar vortex suffered a disturbance called "sudden stratospheric warming" in early January.
The sudden warm spell changed the direction of winds in the stratosphere, and broke the polar vortex up into pockets that drifted down to Southern Canada and the U.S., Kolonjari posted on the CREATE Arctic Science blog.
The polar vortex pockets dragged with them frigid Arctic air that slammed eastern North America with wind, snow, ice and unusually cold temperatures.
It's not in itself unusual for pockets of polar vortex to drift down into Southern Canada — a phenomenon known as "polar intrusion events," said Kolonjari in an interview with CBC News.
"The thing that made this winter different was the amount that happened," she said. "We had extended periods of this vortex air sitting over the southern latitudes, and that is uncommon."
Those who suffered through that long winter will be happy to know that there is a bright side — the lack of a stable polar vortex means ozone-depleting chemicals didn't have time to gather this winter and weren't expected to take a big bite out of the ozone layer this spring.
"We wouldn't expect any significant depletion," Kolonjari said. "It's probably going to be about a typical year."