Most of the United States and parts of Canada have been hit by a heat wave over the weekend, meteorologists say.

The extremely hot weather could affect about 200 million people in major cities like New York, Washington and Boston in the East Coast, and the Midwest region too.

In some places, temperatures could be close to or exceed 100F.

According to experts, the more frequent heat waves in recent years are linked to climate change.

According to new data, the world experienced its hottest June on record this year, with an average temperature worldwide of 61.6F (16.4C).

Earlier this month, Alaska, part of which lies inside the Arctic Circle, registered record high temperatures.

The heat wave is hitting an area stretching from the Central Plains of Colorado and Kansas, to the Great Lakes in the north-east.

Temperatures are also rising in most areas of the East Coast.

New York Mayor Bill de Blasio declared “a local emergency due to the extreme heat” in the city.

In a video posted on Twitter, the mayor said: “This is a heat wave coming up these next days. It’s serious stuff.

“Friday is going to be bad. Saturday is going to be really, really bad on through Sunday.”

Bill de Blasio urged New Yorkers to take the threat seriously, to stay hydrated, and to not go out in the hot weather. He added that 500 “cooling centers” were being opened across New York.

Similar measures were being taken in Detroit and other cities.

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In Canada, heat warnings have been issued for parts of the provinces of Quebec, Ontario and Nova Scotia. Daytime temperatures in Toronto could feel like 104F on July 19 and 20, when humidity is factored in. Severe thunderstorms are also expected.

Canadian meteorologists say that in Montreal the temperature could feel like 113F with humidity over the next two days.

US meteorologists say the heat wave is expected to continue through the weekend, and some cities may see their highest temperatures in years.

In some cities it might feel as hot as in California’s Death Valley – a desert region known for its extreme heat. But they say a dramatic change is on the way early next week, when it is expected to suddenly cool off.