Large swathes of the US and Canada are facing yet another heatwave bringing extreme temperatures and poor air quality.

Toronto and other Canadian cities as well as parts of the US Midwest and Northeast will see the high temperatures that started on Tuesday continue into Wednesday. Further west in the US, millions of Americans already braved the heat.

Millions of people are under advisories in Boston, New York City, Philadelphia and as far west as Buffalo, New York, through Wednesday.

The high temperatures come not long after a record-breaking 4 July heatwave hit large portions of the US, leading experts to warn people to exercise caution.

A strong area of high pressure in the upper levels of the atmosphere is building eastwards across the US and parts of Canada.

This dome of hot and humid air has already led to all-time temperature records being broken in parts of Montana and Utah, and now the heat is shifting eastwards.

The northern Plains, Midwest and Northeast will bear the brunt with temperatures in Chicago forecast to reach 97F (36C) on Wednesday.

At the same time, New York could reach 100F while Washington DC is expecting 102F. A few places could get even hotter, with high humidity making it feel even more uncomfortable.

Central and eastern areas of Canada have also been hot, but a cold front is set to bring lower temperatures over the next few days. It will also bring the threat of severe thunderstorms, which could also affect parts of New England.

Those thundery conditions will develop more widely across the east of North America by the weekend, pushing away the worst of the heat - although temperatures in many areas will remain above the seasonal norm.

Earlier this month, extreme heat disrupted Fourth of July celebrations across parts of the US, including the Washington DC parade, which was cancelled.

More than 165 million people were sweltering under record temperatures along the US East Coast and Midwest. At least 44 deaths across the US were reported as a result of the 1 July to 4 July heatwave.

Heatwaves have become more frequent, more intense and longer-lasting because of human-induced climate change. The world has already warmed by about 1.1C since the industrial era began and temperatures will keep rising unless governments around the world make steep cuts to emissions.