image copyrightReuters

A blistering heatwave has hit Canada and parts of the US, sending temperatures to dangerous highs of nearly 50C (122F).

Dozens of people have died.

Here's what we know about what is going on.

How bad is it?

Temperature records have been shattered across western Canada and the US Pacific Northwest.

Canada broke its country temperature record for a third straight day on Tuesday – 49.6C (121.3F) in Lytton, British Columbia.

Before Sunday, temperatures in the country had never passed 45C.

The heat has been blamed for helping cause the deaths of dozens in the Vancouver area.

The US north-west has also seen record highs – and a number of fatalities.

Portland, a city with a famously rainy climate, broke its all-time high temperature record for three days in a row.

The temperature at Portland International Airport peaked at 46.1C on Monday, going above the previous day's high of 44.4C and Saturday's 42.2C, according to the US National Weather Service.

At least a dozen deaths in Washington and Oregon are believed to be linked to the heatwave.

How have people been affected?

The searing temperatures have left many vulnerable people struggling in the sweltering heat.

The region's climate is typically mild, and many homes do not have air conditioning, which might help explain the sudden rise in deaths.

Many have been forced to take refuge in cooling centres – air-conditioned buildings, such as stadiums, where residents can work and sleep.

image copyrightGetty Imagesimage captionA cooling centre for residents hit by the heatwave

People and infrastructure in urban spaces, absorbing more heat than greener, rural areas, have been particularly affected by the high temperatures.

The heat has been so intense it has melted power cables and buckled roads.

Here's a look at the work happening on SR 544, milepost 7 near Everson. Crews are working on repairs and have the highway closed at this time. pic.twitter.com/7buT2X0YRa

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Vaccination centres have been forced to close or relocate, schools have shut their doors and some public transport has been suspended – including the Portland Streetcar Service.

Elsewhere, shops have sold out of portable air conditioners, fans, ice and water.

What is causing it?

The persistent high temperatures are the result of what is known as a "heat dome" – a mountain of warm air pressing down across a huge area and refusing to move.

As warm air tries to rise, this high pressure system above pushes the warm air back down to the surface. It becomes denser and hotter as it gets compressed.

This mass of hot air is also bending the jet stream – a current of fast-flowing air high in the Earth's atmosphere – around it and preventing cooler weather from moving in.

Experts say climate change is expected to increase the frequency of such extreme weather events, such as heatwaves. However, linking any single event to global warming is complicated.

How long is it going to last?

Weather forecasters predict the heat will now start to ease near coastal regions, and in Seattle and Portland as temperatures start to dip.

But the record-breaking heat is expected to last all week in the interior areas of the Pacific Northwest.

How can I keep cool in a heatwave?

You should cool off immediately if you have the following symptoms: headaches, feeling dizzy, loss of appetite, nausea, excessive sweating, cramps, fast breathing and intense thirst.

If your body's temperature hits 40C (104F), heatstroke can set in, which requires urgent medical help. Danger signs include sweat stopping – the person may feel hot, but dry – and breathing difficulties.

  • How to look after yourself in hot weather

By Mike Hills, Sana Jasemi, Lucy Rodgers, Joy Roxas, Paul Sargeant, Nassos Stylianou, Alison Trowsdale,