US-Canada heatwave: Visual guide to the causes

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

Hundreds 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 three straight days on Tuesday - 49.6C (121.3F) in the village of Lytton, British Columbia.

A wildfire has now burned 90% of the village and damaged critical infrastructure, local officials say.

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

The heat has been blamed for helping cause the deaths of hundreds in British Columbia.

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.

Dozens of 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.

A 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.

In Vancouver, residents reported car windows cracking and melting, even when they were not parked in the sun.

Vaccination centres were forced to close or relocate, schools shut their doors and some public transport was suspended - including the Portland Streetcar Service.

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

There are now fears of more devastating wildfires, such as that seen in Lytton, British Columbia.

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.

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.

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?

Temperatures have been easing in coastal areas of Canada and the north-west US but there has not been much respite for those living in inland regions.

The weather system is now moving eastwards over Canada's Prairie provinces - Alberta and Saskatchewan and parts of Manitoba have been placed under heat warnings.

This mass of hot air has lasted so long because it is strongly diverting the jet stream - a current of fast-flowing air high in the Earth's atmosphere - northwards and around itself, which in turn holds the heat dome in place.

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.