Washington (United States) (AFP) – A record-smashing heat wave was spreading Tuesday from the Mountain West toward the eastern United States and Canada, placing more than 100 million people under heat alerts.

Hot and dry conditions also contributed to fierce wildfires in southwestern Ontario and northern Minnesota, with air quality expected to worsen over the Midwestern and Northeastern United States in the coming days.

The phenomenon known as a heat dome has already shattered all-time temperature records in western states, including 111F (44C) in Billings, Montana, where the previous record was 108F, and 109F in Salt Lake City, where the previous one was 107F.

Extreme heat and humidity persist in the Mountain West but are now spilling over to the densely populated East Coast, which also faced brutal temperatures earlier in the month, as well as Canada’s Ontario and, to a lesser extent, Montreal.

“Above average temperatures and dangerous levels of heat are forecast to enter the Northeast on Tuesday before the most intense heat occurs on Wednesday and expands into the Mid-Atlantic,” said the US National Weather Service.

US cities from Richmond, Virginia, to Boston, Massachusetts, are set to see temperatures soar from the upper 90s to near 100 degrees, with daily high records under threat.

In Ottawa, the federal capital of Canada, and Toronto, highs are expected to reach 100.4F on Tuesday, with temperatures feeling even hotter because of high humidity levels.

“Hot and humid air can also bring deteriorating air quality and can result in the Air Quality Health Index approaching the high-risk category,” Environment and Climate Change Canada said.

In Montreal, the sky turned yellow on Tuesday morning due to smoke from wildfires burning several hundred miles away in northern Quebec and northwestern Ontario.

Cooler temperatures are expected to follow by the end of the week, though the heat dome will persist in the US Mountain West for the rest of July.

Heat domes are large areas of high pressure, where sinking air suppresses development of precipitation and clouds, allowing heat to build up over days and weeks.

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