What Is a Heat Wave and What Causes It to Be So Hot?


A thermometer in Death Valley showing a temperature of 130 degrees Fahrenheit (54 degrees Celsius).

Broke a sweat just looking at this.
Photo: Patrick T. Fallon/AFP (Getty Images)

We live in an era of endless heat. Winter or summer, Arctic or tropics, it doesn’t matter. Heat waves have become a fixture of the climate crisis and modern life.

The U.S. has faced record-smashing heat in 2021, including a June that’s now in the record books as the hottest ever. Temperatures across the country were 4.2 degrees Fahrenheit (2.3 degrees Fahrenheit) higher than normal, and dangerous heat has repeatedly gripped the West.

The impacts of this year’s heat, let alone the heat of years to come, are profound. Hundreds of people died in the Pacific Northwest as did an estimated 1 billion sea creatures amid record heat. Reservoirs have shrunk. Infrastructure literally melted.

The future heat waves will only be worse. That makes understanding them ever-more crucial, from what exactly a heat wave is (a surprising tricky proposition!), how to predict them further in advance so people have warnings, and just what’s in store.

What Is a Heat Wave?

A heat wave is when it’s freaking hot out for a long period of time, of course. But that doesn’t quite cut it from a meteorological standpoint. (Seriously, imagine the record books if the internet had its way.)

“That being a simple question, there surprisingly is not a simple answer,” said Karen McKinnon, a researcher at the University of California, Los Angeles.

Despite heat being the top weather-related killer in the U.S., there’s no standard heat wave definition or threshold. Certain temperature marks can trigger heat warnings or advisories from the National Weather Service on any given day or even days. But when it comes to nailing down a heat wave, the agency’s formal glossary contains this definition:

“A period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days.”

Compare that to the detailed entry on “heavy freezing spray” a few spots down in the glossary, which reads, “An accumulation of freezing water droplets on a vessel at a rate of 2 cm per hour or greater caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.” OK then.

McKinnon noted there are a few other ways to think about what constitutes a heat wave beyond temperatures being hotter than normal for two days or more. For researchers, it can be helpful to think of what percentile a period of extreme heat falls into. That makes it easier to parse data. A lot of how we talk about heat waves also focuses on daytime highs, but she noted the overnight lows are just as important to look at.

“Especially for human health, it’s really important for our bodies to be able to cool off at night,” she said. “And so people who focus more on human health, definitions of heat waves will often think about what’s happening to the nighttime temperatures as well. One way to think about it is that you might want to define a heat wave for the input you’re interested in. So a heat wave for human health could be different from, say, a heat wave in terms of impact on crops.”

How Do Heat Waves Form?

Well, depends on how long you got. The short answer is, lots of ways. The longer answer is it depends on how in-depth you want to go.

Areas of high pressure are frequent heat wave culprits, particularly in summer. They can lock in persistent sunny skies and actually intensify as heat radiates off the ground, locking in more high pressure. There’s even a term for it: heat domes. This is the setup that led to the deadly Pacific Northwest heat wave in June 2021. But local topography can also play a role.

“In the PNW, the Cascade Mountains helped to boost temperatures as warm air ‘downslopes’ off the mountain thanks to the east wind from the clockwise high pressure,” Kathie Dello, the North Carolina state climatologist, said in a Twitter DM.

Sometimes wild zigs in the jet stream can also help bring heat streaming up from the lower latitudes to cooler locales to the north (or south depending on which side of the equator you’re on). In fact, when the jet stream gets really wavy, it can actually help researchers predict just where heat waves will form. You can imagine the jet stream like the battle ropes at a gym that folks use to work out their arms. When you use those ropes, it can send oscillations from your flailing arms down the line, and you can control whether they make big oscillations or small ones.

So it is with the jet stream. All sorts of perturbances, whether it’s a tropical cyclone in the western Pacific or a big area of high pressure somewhere else, can cause the jet stream to oscillate like the gym ropes. And those oscillations set up into predictable numbers. That’s often why you’ll have various hot spots around the globe. During the Pacific Northwest heat wave, for example, there were also scorching temperatures in parts of Europe.

“Extremes are not singular,” McKinnon said. “They tend to be co-located.”

Great news for forecasters, but perhaps bad news for disaster managers (or your humble extreme weather correspondent) since it means having to potentially juggle multiple crises at once.

Climate Change Amplifies Heat Waves

It’s perhaps not a shocker that climate change (aka global warming) leads to worse heat. But it might be surprising that the roughly 1.8 degrees Fahrenheit (1 degree Celsius ) of warming has had an outsize impact on heat waves frequency and severity.

“Our background state is shifting,” Dello said. “So when we talk of warming of a few degrees on average, we’re getting there by having more hot days. As we continue to add heat trapping gases to the atmosphere, we’re seeing just how sensitive the climate is and having unprecedented heat.”

Scientists often invoke this idea that climate change is like loading the dice that makes it more likely that extreme weather will happen. I’d like to propose a new analogy, though. It’s instead like swapping out the dice completely. Instead of offer 1 through 6 on a six-sided die, our new dice runs from 2 through 7, and they’re loaded to boot. The Northwest heat wave in late June and early July of 2021 neatly illustrates that. A recent snap analysis shows it was a 1-in-150,000-year event without climate change. It was 1-in-1,500-year event in our current climate, making the all but unimaginable a now fringe event. If the world manages to somehow keep heating to the Paris Agreement threshold of 2 degrees Celsius (3.6 degree Fahrenheit), the event will have a likelihood of happening every five to 10 years. It’s mind-bending to think about.

“Climate change is causing us to obliterate our heat records, and these will be some of the cooler summers of the 21st century if we don’t act on climate,” Dello said.

We Urgently Need to Adapt to Living With Heat Now

Given the risks that played out in the Northwest and have played out elsewhere from unprecedented fires in Australia and Siberia last year to heat eroding sea ice in the Arctic, it’s clear we have our work cut out for us. First and foremost, cutting emissions is vital.

But so, too, is adapting to heat waves. There are some success stories to look at, though they all came after tragedies. In 2003, a heat wave killed an estimated 70,000 in Europe. Governments responded with more user-friendly warning systems and other heat plans, and in subsequent years, even worse heat waves have left fewer dead.

The Northwest is still counting the dead, but it’s increasingly clear that many were poor, old, and/or alone when they died. More cooling centers or programs that subsidize air conditioning for those in need are one way to help. Low-income households, particularly Black, Latinx, and Indigenous ones, spend four times as much on utilities as their wealthier counterparts. Clearly the playing field needs to be leveled. Social programs could also play a role. New York recently implemented a pilot program to “promote community cohesion” by ensuring those who were homebound had someone checkup on them during extreme heat. Mutual aid groups also played a key role in responding to the Pacific Northwest heat.

But individual-level fixes aren’t enough. Our cities need a major overhaul for heat. That includes more greenspace to combat the heat island, particularly in poorer neighborhoods and communities of color. Redlined communities, for example, face more extreme heat. Given the energy burden Black and brown communities bear along with extreme heat, it’s more vital than ever that any plan to help keep people cool is a just one.





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