What to Know About the Heat Waves

In most parts of the United States, temperatures must be above the historical average in an area for two or more days before the label “heat wave” is applied.

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In the summer of 2021, few places in the United States have been spared from enduring back-to-back days of punishing heat.

Much of the Pacific Northwest was roasted by a stretch of triple-digit temperatures in June. The Northern Rockies were blasted with hot air, too. And this week, millions of people in states from Montana to Georgia have sweltered through stifling heat.

Here is a guide to heat waves, and the heat domes that often cause them:

What causes a heat wave?

In most parts of the country, temperatures must be above the historical average in an area for two or more days before the label “heat wave” is applied to a hot spell, according to the National Weather Service. But the definition can vary by region; in the Northeast, it is defined as three straight days in the 90s or above.

Heat waves begin when high pressure in the atmosphere moves in and pushes warm air toward the ground. That air warms up further as it is compressed, and we begin to feel a lot hotter.

The high-pressure system pressing down on the ground expands vertically, forcing other weather systems to change course. It even minimizes wind and cloud cover, making the air more stifling. This is also why a heat wave parks itself over an area for several days or longer.

What is a heat dome?

As the ground warms, it loses moisture, which makes it easier to heat even more. And in the drought-ridden West, there is plenty of heat for the high-pressure system to trap.

As that trapped heat continues to warm, the system acts like a lid on a pot — earning the name “heat dome.” In the drought-ridden West, there was plenty of heat for the high-pressure system to trap, leading to triple-digit temperatures in late June and early July that killed hundreds of people in Oregon, Washington and British Columbia.

The deadly weather event would have been all but impossible without climate change, according to a team of researchers.

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The Oregon Convention Center served as a cooling station in Portland in June, when the city reached a record-high temperature.Credit…Kathryn Elsesser/Agence France-Presse — Getty Images

Why is it hotter than normal in North America?

We have long known that the world has warmed by more than 1 degree Celsius (about 1.8 degrees Fahrenheit) since 1900, and that the pace of warming has accelerated in recent decades. The warmer baseline contributes to extreme-weather events and helps make periods of extreme heat more frequent, longer and more intense.

Last year was the warmest on record, and the past seven years have been the warmest in recorded history, making extreme summer heat more frequent.

How can I safeguard against possible power outages?

To help reduce demand, consider raising the thermostat by a few degrees and closing window shades and blinds. Avoid using large appliances like ovens, washing machines and dryers during the hottest part of the day, and turn off all lights and electronic devices not in use. With water heating accounting for about 18 percent of the energy consumed in your home, consider shorter or colder showers, the Energy Department suggests.

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