As some parts of Canada set heat records of more than 45C, Yellowknife residents woke on Tuesday to a heat warning for the city – though the forecast currently doesn’t call for more than 29C.
While that’s warm, it’s not particularly close to Yellowknife’s all-time record high of 32.5C (set in 1989) and nowhere near some of the week’s forecast highs elsewhere in the territory, such as 38C now expected in Fort Smith on Wednesday.
So why the heat warning?
The answer is that Environment and Climate Change Canada’s heat warnings use different criteria depending on where in the country you live.
For example, in parts of southern British Columbia, the temperature needs to reach 35C for a warning to be triggered. In Yukon or Newfoundland and Labrador, it’s 28C. A map published by the federal government shows how the criteria change from region to region.
But the rules governing when warnings are published are also a little more complex than simply whether a certain temperature is expected.
In the Northwest Territories, a heat warning is issued “when two or more consecutive days of daytime maximum temperatures are expected to reach 29C or warmer and nighttime minimum temperatures are expected to fall to 14C or warmer,” Environment and Climate Change Canada says.
In other words, sustained heat at night is just as important as the daytime high, and the conditions must be expected to last for at least two days.
The forecast for Yellowknife, as of Tuesday morning, just-about meets the above criteria for daytime highs and easily exceeds the 14C barrier for nighttime highs, with a stuffy 21C expected overnight on Tuesday into Wednesday followed by 18C the following night.
If you live in Nunavut, there is no formal point at which a heat warning is activated. Nunavut and Nunavik are the only parts of Canada where no heat warning program exists.
Meanwhile, winter extreme cold advisories work in the same way: the trigger point changes for different parts of Canada.
That’s why parts of Ontario get extreme cold warnings for -30C, but the NWT only activates extreme cold warnings for -50C or below (including wind chill).
Why issue these warnings in the first place?
Environment and Climate Change Canada says they’re used to raise awareness and initiate preparations for action where necessary, particularly by levels of government.
For example, extreme cold warnings may serve as prompts to increase capacity at shelters.
This week’s Yellowknife heat warning is part of a broader range of warnings issued for much of the NWT. Many areas of the territory are set to experience temperatures well above 30C, with some pushing above 35C.
“A prolonged and dangerous heat wave will continue this week for parts of the Northwest Territories. Sections of the Mackenzie River well begin to moderate on Wednesday, while regions near Great Slave Lake will moderate later this week,” Environment and Climate Change Canada said.
“Daytime high temperatures will climb into the mid to high 30s for parts of the territory. Little to no reprieve from the heat is expected, as overnight lows will remain between 15 and 20 degrees Celsius.
“The duration and magnitude of this heat event will lead to increased risk of heat-related illness.”