What to Know About Heat Waves

The summer of 2024 is shaping up to be a hot one. Most of the United States can expect a summer with above normal temperatures, according to the National Oceanic and Atmospheric Administration’s seasonal outlook.

Here’s what to know about heat waves and their causes:

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 often 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 summer of 2021, 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.

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 main culprit of global warming today is humans burning fossil fuels. Thousands of scientists, who have studied the causes across decades, have reached this overwhelming consensus. Globally in 2022, humans emitted roughly 36.8 billion metric tons of planet-warming carbon dioxide by burning coal, natural gas and oil for energy.

The warmer baseline contributes to extreme-weather events and helps make periods of extreme heat more frequent, longer and more intense.

What can you do to stay safe in the heat?

Dr. Maria Raven, chief of emergency medicine at the University of California, San Francisco, offers some tips for staying safe if you are going to go outside in the heat.

• Give yourself time to acclimate: Dr. Raven says it takes a week or two to get used to extreme heat. Increase the amount of time you spend outdoors each day gradually, if you can, by about 20 percent.

• Go outside in the morning or evening: Even a five- or 10-degree temperature drop can make a big difference.

• Know the signs of heat exhaustion and heatstroke: If you’ve got heat exhaustion, you’ll be sweating profusely, and you may feel a little nauseated. Your skin may be red and hot to the touch, as if you have a fever. If your body approaches heatstroke, which is severe enough to require medical attention, you will stop sweating, and your core temperature will elevate quickly.

• Know what to do if you’re suffering from heat-related illness: The top priority, Dr. Raven said, is to hydrate. Drink water. You can also use ice packs (in the groin or armpits) and sit near a fan if possible.

• Don’t push yourself, or anyone else, past comfort: “It can be a badge of honor to go and work out when it’s really hot, but it’s not worth it,” Dr. Raven said. That includes student athletes and employers. It’s crucial to give everyone who is outside in the heat time to rest and drink water.

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.

Jill Cowan contributed reporting.