A rare heat wave has shattered records across the U.S. Pacific Northwest and Western Canada. In a region of the world generally known for cool rain and cloudy skies, the high temperatures have buckled highways and triggered rolling blackouts. The swelter is caused by a heat dome, a weather phenomenon that can trap an entire region under searing skies for days.

1. What’s a heat dome?

When the summer sun warms air above the ground or ocean, that air can then rush up into the atmosphere to form a mountain -- or dome -- of slow-moving hot air under higher pressure that blocks new weather systems from moving in. Basically, it’s a mass of warm air that’s stuck over a certain area. Heat domes can actually force the jet stream, bands of strong wind that generally blow from west to east, to flow around it. In the case of the heat dome in the U.S. Northwest, that means the jet stream will flow north, causing it to slow down.

2. What are the effects?

People without air conditioning see temperatures in their homes rise to unlivable levels, and aid groups struggle to provide homeless people with cold water and shade. More than 600 people are killed by extreme heat in the U.S. each year, and many more die in other countries. The mountain of heat bakes the Earth under it, which can damage crops, worsen droughts and dry out vegetation, providing fuel for wildfires. Energy demand rises, lifting rates for natural gas and wholesale electricity as well as taxing supplies, driving some utilities to cut power to customers to ensure the health of the entire grid. Prolonged and intense heat can hobble public transportation, forcing the shutdown of commuter trains and streetcars as cables melt. Railroads often slow to a crawl because sunbaked steel rails expand, kinking lines and causing derailments. Also, heat domes become blocks in the atmosphere, like boulders in a river, bending the flow around them and leading to other consequences, such as stalled areas of low pressure that can bring floods further downstream.

3. Are heat domes becoming more common?

Some research concludes that they are becoming more frequent, but there isn’t a deep body of science on that question yet. Past heat domes have caused overwhelming damage. A heat wave caused by a dome of high-pressure air struck the U.S. Midwest in 1995, killing more than 500 people in Chicago and hundreds more across the region. A heat dome in California last summer broke records across the state; as Californians switched on their air conditioners, the resulting energy spike triggered blackouts that left millions briefly without power.

4. Does climate change play a role?

Certainly climate change makes all kinds of heat waves both more likely and more extreme, although the evidence for heat domes specifically is less strong than it is for extreme temperatures in general. Today, scientists don’t even study whether climate change increased the strength of a heatwave. Instead, they investigate how much it did so. Other effects of climate change work alongside heat domes to intensify the ramifications of extreme temperatures. For example, long-running droughts and increased risk of wildfire -- both climate change hallmarks -- are amplified by the prolonged high temperatures caused by a heat dome.

5. How long do heat domes last?

Typically about a week. Eventually the formation becomes too big to keep standing and falls over, releasing the trapped air and ending the swelter.

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