Imagine a massive sports stadium the size of a Canadian province pumped full of dry, sweltering heat and plunked overtop a sweeping stretch of North America.

That, in a nutshell, is a so-called "heat dome" – and it’s the main reason for the scorching temperatures above 30 C across central Canada, the Maritimes and 26 U.S. states this weekend.

The unusual weather phenomenon is like a “lid” that seals hot air in a region over several consecutive days, explained Environment Canada senior climatologist David Phillips. Without a cold front to pop the dome, the same hot air is recycled within the dome day after day.

“Inside that dome, there’s really no weather in a way. There’s no precipitation, it’s just hot, it’s dry, the sun is baking the ground, and even when the warm air rises it kind of is compressed and returns back,” Phillips told CTV News Channel on Saturday.

“So it becomes very stale air and the longer you have this dome over you, you’re breathing the same air yesterday that you breathed maybe tomorrow, and it just progressively keeps get warmer and drier.”

Environment Canada issued at least 16 heat warnings across southern Ontario on Saturday afternoon and included warnings for Toronto, Niagara region, Waterloo and Hamilton. On Friday, several communities across New Brunswick were subject to heat advisories. In parts of the central U.S., temperatures reached the mid-40s.

The above-30 C heat is expected to remain in central Canada until at least Tuesday.

And while heat domes are uncommon, Phillips said they’re not exactly rare.

“You see them every year. We’ve had so many bouts of it this year. For example, in southern Ontario we’ve had maybe 20 days where we’ve had temperatures about 30 C. It began in May, there were several in June and now in July. Last year at this time we had two of those kind of days,” he said.

The circulation of the same hot air could pose a health concern for people with respiratory problems, especially for those in areas with air pollution, Phillips said. He added that seniors, infants, the homeless and people who work outdoors are the most vulnerable to overheat amid the warm conditions.

“Even the fit and athletic people, when you get temperatures this high … you don’t want to be out in the midday sun, because what we’re talking about here are temperatures that are low- to mid-30s, and we’re talking about shaded temperatures. You go out in that sun, if you’re exposed to that sun, you can add another eight, nine degrees to it,” he said.

For those in need of reprieve from the heat, Environment Canada advises to take cool showers or baths and prepare meals that don’t need to be cooked in the oven. The weather agency also says it’s important to visit elderly friends and family, especially those who are chronically ill, to make sure they stay cool and hydrated.

Environment Canada also says pets and people should never be left in a parked car or in direct sunlight.