Huge tracts of North America are sweltering under very high temperatures, which are close to 50ºC in some places. The phenomenon behind this weather is known as a heat dome and the one that's currently settled across parts of Canada and the US is feared to have caused the death toll to rise in British Columbia.

To understand what a heat dome is, how it is caused and what effects it can have, here are three heat dome facts.

1. What is a heat dome and what causes it?

There are two things that contribute to a heat dome – atmospheric pressure and rising sea temperatures.

High pressure way up in the atmosphere can trap heat, leading to an increase in temperatures at ground level. This effect is fuelled by heat rising from the ocean, creating an amplification loop. The high pressure in the atmosphere acts like a dome and prevents the heat from dissipating. As the air gets hotter, everything gets hotter and hotter.

a diagram showing how life-threatening heat gets trapped by high pressure

Life-threatening heat gets trapped by high pressure.
Image: NOAA

“When strong, high-pressure atmospheric conditions combine with influences from La Niña … vast areas of sweltering heat (get) trapped under the high-pressure ‘dome’," according to the US National Oceanic and Atmospheric Administration (NOAA).

In recent decades, the NOAA says, temperatures have risen more in the western Pacific than in the eastern Pacific. Some of the rising hot air is transported toward land by the jet stream, high in the atmosphere. Eventually it sinks, creating a dome and generating heat waves.

2. What effect is the heat dome having?

"This is life-threatening heat," Multnomah County Health Officer Jennifer Vines said in a statement reported by Reuters.

Portland, Oregon, which has a population of 654,741 people, is in Multnomah County. The region has seen a relaxation in pandemic-related crowd control measures in theatres, swimming pools, and shopping malls.

A number of emergency cooling shelters have been opened so that people without domestic air conditioning units can escape the heat.

One of the biggest health risks in a heat wave such as this one is heatstroke. Unless detected and treated promptly, heatstroke can lead to many serious problems including damage to the brain, heart, muscles and internal organs. Commonly accompanied by feelings of confusion and delirium, heatstroke can trigger seizures and lead to comas, brought on by swelling of the brain and organs.

Older people are more susceptible to heatstroke, as the body’s ability to regulate heat deteriorates with age.

3. How can people look after themselves in extreme heat?

When the outside temperatures are so high they threaten people’s health, it is safer to stay indoors. Fans can help alleviate some of the discomfort, but they have limited benefit because they don’t cool the air.

Air-conditioned rooms and buildings are the biggest help to people caught under a heat dome. In many parts of the US and Canada, air-con is commonplace in stores, offices, and homes. But this most recent heat wave has hit population centres that are used to much milder weather.

Unlike southern destinations like California or Texas, the cities of Portland and Seattle are more used to rainy, milder weather. Portable air-con units, along with fans and even ice, were said to have sold out quickly in Portland.

The medical advice for anyone experiencing dangerously hot weather is to wear loose-fitting, lightweight clothing, and stay well hydrated. People have also been advised not to exert themselves, and never to leave anyone in a parked car – that includes children and animals.

Everyone should pay attention to how they are feeling, especially if age or pre-existing health conditions might put them at a greater risk. “Avoid the heat and act quickly if you notice symptoms of overheating,” says the Mayo Clinic website.