The sweltering heat cutting across southern Ontario this week is fuelled by hot air currents flowing north across the border from the United States, Canada's top weather forecaster said.

The heat reached the mid-30s Tuesday for much of the province stretching from the Niagara region up across Lake Ontario to Ottawa, but felt more like 40 degrees due to elevated humidity levels.

"In the first part of this week we're seeing the American air…It is warm, it's humid and the air quality is not necessarily pristine fresh," said David Phillips, senior climatologist at Environment Canada.

"It's very muggy (and) oppressive morning, noon and night."

But don't fill the freezer with ice just yet, Phillips said.

"Canadian air" will return and drop temperatures by Friday, when the winds are expected to shift and blow in from the north, bringing cool air that will push temperatures down to the teens and low-20s.

"We're going to see an end to this heat wave — this oppressive kind of sultry weather…it's going to be replaced by something that is going to be gorgeous.

"What we're going to see is almost Canadian air…Arctic air. It's going to be fresh, it's going to be near-normal temperatures (or) slightly below."

The combination of the quick change between hot and cool air in one week is rare, Phillips said.

People in areas of the province affected by the heat wave should avoid any strenuous outdoor activity until the temperature drops, he said.

"If you've got some things to do right now that are going to exert yourself…then hey — postpone those, wait 'til Friday or Saturday.

"It's going to be much more comfortable, much more healthy for your body to engage in physical activity."