Extreme heat might not sound 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. And by far.

Of course, a heatwave in one state might not be a heatwave in another; the definition is usually 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 heatwave in Tucson is higher than the threshold in Seattle.

As Umair Irfan pointed out for Vox, heatwaves 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 also dries out.

The sinking air acts as a cap or a heat dome and traps 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 — is then free to bake, and in the long days and short nights of summer, heat energy quickly accumulates and temperatures climb.

Heatwaves typically last around five days, but can stick around for longer if the high-pressure system is blocked in place. “In some cases, you actually can get these kinds of patterns getting stuck, and that can lead to heat waves lasting much longer,” Karen McKinnon, an assistant professor of environment and sustainability at the University of California Los Angeles, told Vox.

Eventually, the high-pressure system will weaken, allowing in cooler air and precipitation that can bring the heatwave to an end. However, as the warm season continues, more high-pressure systems can settle in and restart the process.