4 things to know about the 'oppressive' heat wave in Ontario, Quebec and Atlantic Canada

Millions of people are under a heat warning in Ontario, Quebec and the Atlantic provinces, with Environment and Climate Change Canada warning that temperatures could reach as high as 35 C.

The weather service is warning of "dangerously hot and humid conditions" in parts of Ontario and Quebec in particular.

Here's what you need to know.

How hot will it get?

Humidex values, which combine the air temperature with humidity to calculate what heat feels like to the average person, could reach 45 C in parts of Quebec and Ontario.

The overnight lows are expected to remain above 20 C.

Parts of New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland and Labrador are also under a heat warning. Other parts of the country fartherwest, meanwhile, are experiencing unseasonably cool weather.

DavePhillips, senior climatologist with Environment and Climate Change Canada, described the system as a massive heat dome, which is a high-pressure system that works to trap high temperatures near the Earth's surface. The heat rises and then gets pushed back down, like a convection oven, he said.

"It's almost going to be what you see in a jungle kind of situation with the tropical nights," he said in an interview, warning that the heat could be "very oppressive."

WATCH | What is a heat dome? A climatologist breaks it down:

What is a heat dome? A climatologist breaks it down

2 months ago

Duration 2:32

Dave Phillips, senior climatologist with Environment Canada, explains what conditions create a heat dome and how 'oppressive and sultry' temperatures get locked in for days.

How long will it last?

The heat wave is expected to last through Thursday evening, with highs expected a day prior.

The mass of warm air is coming from the United States, where more than 70 million people are under heat advisories or watches on Monday, stretching from the Southwest northward up through Denver and into Chicago.

Is this unusual?

Heat waves are becoming increasingly extreme as the climate warms, experts say. A rapid forecast analysis by Climate Central, a U.S.-based research group, estimated that the high temperatures in the eastern U.S. forecast for Tuesday to Thursday were made twice as likely due to climate change.

"Human-caused climate change makes the heat season start earlier and makes heat waves hotter and more dangerous," said Andrew Pershing, director of climate science.

WATCH | Temperatures to soar this week:

Heat warnings across Ontario, Quebec, N.B., N.S.

2 months ago

Duration 1:46

CBC News meteorologist Colette Kennedy outlines the heat dome that is setting in across eastern North America. Thermometre temperatures are expected to reach the low 30s C with humidex into the 40s.

A report last month by the same group, along with World Weather Attribution and the Red Cross Red Crescent Climate Centre, examined 76 extreme heat waves across 90 countries over a 12-month period starting May 15, 2023.

The report says in that period, 6.3 billion people roughly 78 per cent of the population experienced at least 31 days of extreme heat that were "made at least two times more likely due to human-caused climate change."

Environment and Climate Change Canada is forecasting higher than normal temperatures throughout most of Canada this summer, after the hottest year on record.

How can I stay cool?

Extreme heat is a major health risk, particularly for older adults, infants and young children, and people with disabilities or mobility issues. Here are some tips to stay cool:

• Drink plenty of water, even before you feel thirsty, to avoid dehydration.

• Find a place with air conditioning to hang out, like a shopping centre, library, community centre, swimming facility or grocery store. Many cities offer cooling centres during periods of extreme heat.

• Be on guard for symptoms of heat-related illnesses, which include heat stroke, heat exhaustion, heat rash and heat cramps.

• Avoid intense or moderately intense physical activity, especially during 11 a.m. to 4 p.m. All workers should take regularly scheduled breaks in a cool or shaded space.