Heat domes, heat islands, mega-droughts, and climate change: the anatomy of worsening heat waves.

The Pacific Northwest is sweltering under a record-breaking heat wave. Portland reached 116 degrees Fahrenheit this week. Seattle reached 108 degrees. Vancouver reached 89 degrees. The searing heat has buckled roads, melted power cables, and led to a spike in deaths. It’s especially concerning in a region like the Pacific Northwest, where few buildings have air conditioners.

This follows weeks of extremely high temperatures across the Northern Hemisphere and an early-season heat wave in North America that triggered heat warnings for 50 million people. 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. The forces behind them are complex and changing. They’re a deadly public health threat that can exacerbate inequality, cause infrastructure to collapse, and amplify other problems of global warming. Even more worrying is that in the context of the hot century ahead, 2021 may go down in history as a relatively cool year.

Heat waves, explained

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

Heat waves 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 acts as 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 with so 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.”

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