How heat waves form, and how climate change makes them worse

It’s an alarming echo of 2023, which was the hottest year on record, but this year could be hotter still. Though the Pacific Ocean is shifting into its La Niña phase, which typically brings cooler global weather, the extraordinary warmth over the past year is still baked in. Scientists say these record highs align with their expectations for climate change, and warn that more scorchers are coming.

There’s more to heat waves like this than high temperatures, though. The forces behind them are complex and changing. They’re a public health threat that can exacerbate inequality, cause infrastructure to collapse, and amplify other problems stemming from warming. But with global average temperatures continuing to rise, more records will fall.

Extreme heat might not seem as dramatic as hurricanes or floods, but the National Weather Service has deemed it the deadliest weather phenomenon in the US over the past 30 years, on average.

What counts as a heat wave is typically defined relative to local weather conditions, with sustained temperatures in the 90th to 95th percentile of the average in a given area. So the threshold for a heat wave in Tucson is higher than the threshold in Seattle.

During the summer in the Northern Hemisphere, the northern half of the planet is tilted toward the sun, which increases daylight hours and warms the hemisphere. The impact of this additional exposure to solar radiation is cumulative, which is why temperatures generally peak weeks after the longest day of the year.

Amid the increase in temperatures in the summer, meteorology can push those numbers to extremes.

Heat waves typically begin with a high-pressure system (also known as an anticyclone), where atmospheric pressure above an area builds up. That creates a sinking column of air that compresses, heats up, and oftentimes dries out. The sinking air can actas a cap or heat dome, trapping the latent heat already absorbed by the landscape. The high-pressure system also pushes out cooler, fast-moving air currents and squeezes clouds away, which gives the sun an unobstructed line of sight to the ground.

The ground — soil, sand, concrete, and asphalt — then bakes in the sunlight, and in the long days and short nights of summer, heat energy quickly accumulates and temperatures rise.

Heat waves are especially common in areas that are already arid, like the desert Southwest, and at high altitudes where high-pressure systems readily form. Moisture in the ground can blunt the effects of heat, the way evaporating sweat can cool the body. But when there’s little water in the ground, in waterways, and in vegetation, there isn’t as much to soak up the heat besides the air itself.

“It compounds on itself,” said Jonathan Martin, a professor of atmospheric science at the University of Wisconsin Madison. “When you’re dry, you get warm. When you’re excessively warm, you tend to build and strengthen the anticyclone, which encourages continuation of clear skies, which in turn encourages a lack of precipitation, which makes it drier, which makes the incoming solar radiation more able to heat the ground.”