The first heat wave of Summer 2025 is about to hit Eastern Canada, with temperatures climbing to the mid-30s early next week. Add the humidity from a wet spring and it will feel even hotter.
We’d better get used to it, as it is signalling “a torrid summer ahead,” says David Phillips, senior climatologist with Environment and Climate Change Canada.
“We rarely go a year without a heat wave,” he says, “but this will be the first one and people have to learn how to deal with it again.”
The high pressure system will settle over southern and eastern Ontario as well as southern Quebec. It will edge into southeastern Manitoba on one side and western New Brunswick on the other.
What is such a heat event called?
Contrasting this spate of heat with the renown “polar vortex” of winter, Phillips refers to this phenomenon as a “Bermuda High.” That’s a
semi-permanent, subtropical area of high pressure, which usually migrates east-west, back and forth across the North Atlantic, influencing weather patterns.
Except this one is further west and further north than usual, says Phillips.
“It circulates southern air further north.”
There’s no doubt it will feel oppressive, says Phillips, adding that a heat dome squeezes air molecules, pressing them downward, creating heat from the friction.
How will a lot of spring rain play a role?
Moisture from a particularly wet spring will compound with the heat. There has been 30 to 40 per cent more precipitation than normal this year, he says.
During the day the humidity may make it feel like 40 degrees Celsius or more.
Another factor in the mix is the sunshine. Long summer days “prevent the cool-off,” he says. At night, it will feel tropical, with temperatures above 20 C.
How far east will the heat extend?
The Maritimes won’t get the extreme heat, but temperatures will rise, particularly in western New Brunswick.
What about Western Canada?
Western Canada has been experiencing heat already, with many hot, dry days (made worse by wildfire smoke), he says. But, he adds, in eastern Canada, people have been asking when the warmth is going to arrive.
Normally, Eastern Canada would have several days in the 30s by now, says Phillips, but there were none in May and only one in June. Contrast that with Winnipeg, which he says has had nine or 10.
“In eastern Canada, people have felt left out, on the sidelines.”
How will the heat play out this weekend/next week?
But now summer heat is arriving with a vengeance. It will be a three-day event with temperatures rising to the mid-30s in Toronto, Ottawa and Montreal, accompanied by nighttime temperatures in the 21-23 C range,
with the humidex in the low to mid 40s
. “That’s eight or nine degrees warmer than normal.”
Ultimately, it seems we had better get used to this. All the weather-mapping models from Canada, the U.S. and Europe are predicting increasingly hot summers, says Phillips.
“This will be the first heat wave, but it’s not going to be the last. We’re going to see repetition throughout the summer.”
How does extreme heat affect the body?
He cautions that publicized high temperatures are measured in the shade. As a result, “going out into the sun could add another seven to 10 degrees to the body. It’s a lot of stress on the body. Too much.”
It results in increased hospital admissions, he notes. “People may not be dying in the streets but there is increased respiratory and cardiovascular distress.”
What precautions should you take?
The
urges people to stay indoors between 10 a.m. and 2 p.m., the hottest period of the day. Drink plenty of cool fluids — even if you’re not thirsty. And avoid alcohol and caffeine, as both can result in dehydration.
Check regularly with the children and seniors in your life to ensure they are drinking enough water.
Finally, says the Red Cross, be aware of the following signs of heat illness: dizziness or fainting; nausea or vomiting; headache; rapid breathing or heartbeat; extreme thirst; dark yellow urine; muscle cramps, especially in arms, legs, or stomach.
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