When you can't escape the heat, you're in a 'heat dome'

During a heat wave, it can feel like being trapped by rising temperatures on all sides — and that's basically what's happening to the air in an event that has become known as a "heat dome."

The term is used to describe a "lid" that forms when high pressure hovers in the atmosphere, stopping the hot air from escaping and trapping it at the Earth's surface.

"It's like putting a dome or a lid on a frying pan: the sun comes in and bakes those temperatures at the ground and the air rises," Environment Canada senior climatologist Dave Phillips said in an interview with CBC News Network.

"But the lid, it suppresses or compresses the air, and it sinks to the ground and it warms up even more, and just absolutely creates furnace-like conditions."

Right now, there are at least two of these weather systems over North America, said Phillips. There's one "dome" in the west and another in the east, the latter of which will make its way to southern Ontario and Quebec this weekend.

These types of heat waves dominate all day and are hard to escape, he said.

A "heat dome" isn't a defined meteorological term, said Peter Kimball, a warning preparedness meteorologist with the Meteorological Service of Canada. But it has been used in recent years to talk about intense heat waves. 

"It's basically hot weather," he said. 

The effect occurs when there is little or no variation in the temperature and the heat becomes trapped without any cooler air moving in, Kimball said, which is why the high temperatures can last over days.

High temperatures this weekend

And while the forecast for southern Ontario will definitely be hot this weekend, Kimball said it may not actually break any records.

In Windsor, Ont., for example, it's predicted to be 35 C on Friday. But the city's record-high temperature for that date is 36.7 C, recorded at the airport in 1946, Kimball said.

And the Riverside weather station in Windsor, which has been collecting temperatures for more than 100 years, has the record temperature for July 20 (Saturday) set at 36.7 C in 1926.

"The conclusion from this is that hot air in southern Ontario is not a new phenomenon," Kimball said.

On average, however, global temperatures are on the rise. The U.S. National Oceanic and Atmospheric Administration's latest climate report found that June 2019 was the globe's hottest in the 140 years that records have been kept. 

It's expected that higher temperatures for longer periods of time will become the norm, said Xuebin Zhang, a research scientist with Environment and Climate Change Canada.

"We can attribute this increase to global warming," Zhang said. "The consequence is now we have more heat waves and temperatures are now one degree hotter than in the 1950s." 

In the North, average temperatures have gone up by three degrees, he said.

Northern heat wave

Heat waves are also happening right now in northern areas, Zhang said, but the threshold is different.

"In Nunavut, if it's 20 or 22 C, they might feel it's really hot."

On Thursday, there were several heat warnings in effect for parts of the Northwest Territories, with forecasted temperatures around 29 C.

Earlier this month, Anchorage, Alaska recorded temperatures over 30 C for the first time.

And in Alert, Nunavut, a heat wave earlier this week brought temperatures of 21 C at a time when the average temperature is usually 6 or 7 C.

"That was a three-day event — not just a bubble of heat that lasted for a couple of hours," said Phillips. "A three-day heat wave in the most northerly place in the world."