In the age of climate change, what counts as a heat wave?

A swath of the country from Southern California to Florida has been in the grips of extreme heat for weeks now.

In Phoenix, it hit at least 110 degrees for 19 days in a row — a new record. And Wednesday is expected to be day 20. But as we learn to adapt to the impacts of climate change, what actually counts as a heat wave?

Grist staff writer Kate Yoder reported on what she called a “freak heat wave” in the Pacific Northwest in June of 2021. Temperatures reached 116 degrees in Portland, Oregon, and the heat even melted streetcar power cables there. About 800 people died across Oregon, Washington, and British Columbia at the time.

How is a heat wave actually defined at this point?

“You kind of know a heat wave when you feel one, right? But defining it is actually harder than you'd think. Climate change is raising the bar for what we consider to be really hot.

“At some point, are we going to have to change the definition of a heat wave in a given place, when the extreme heat of the past becomes our average summer weather? And there are other complicating factors for defining a heat wave too, because they're not just determined by a single temperature in itself, but what different weather conditions mean for people's health. Summer weather in Las Vegas, what's normal there would be super hot in Seattle where we aren't adapted to that kind of heat. And some people are more at risk than others, like younger people, older people and people who spend a lot of time outside.”

Is a heat wave a string of days where the temperature is above normal?

“Yeah, the World Meteorological Society's definition is that a heat wave is five days in a row that have a high that's hotter than normal by nine degrees Fahrenheit.

“But by other definitions, it could be as short as two days long, or there could be a different threshold for what temperatures count. And I think another thing to consider is that looking at highs alone ignores other important factors. Summer nights are warming even faster than the days are, and that can leave people without a chance to cool down.”

Why is it important to avoid overusing the term ‘heat wave’?

“With heat advisories, you don't want to be issuing those every single day in the summer, or you risk people not paying attention anymore.

“In terms of extreme weather, heat is the number one killer in the U.S. It's deadlier than hurricanes or wildfires, and it's often called a silent killer because people don't really see it as this threat. They see it as something we all live with every summer.”

How does humidity play a role in extreme heat?

“Sweating is our main biological process that stands between us and this really horrible death by heat stroke, but it does have limits. Very hot and very humid weather can actually make sweating more ineffective.”

How should we alter the way we talk about heat and weather during the summer?

“I do think that this general cultural attitude we have towards heat is leading us to underestimate its danger to us. Last year, some scientists started calling summer danger season instead to direct attention to threats like heat. So I think it's just paying attention to the weather around you and how you're feeling and knowing that heat can creep up on you, even when you're not expecting it.”

Are people there changing the way they think and talk about the weather where you live in Seattle? 

“The adoption of air conditioning here has really skyrocketed in recent years, especially after the heat wave in 2021. Our buildings aren't designed to cool down. They're more designed to keep in the warmth because it's so cold here for so much of the year.

“So there's just different ways that cities can redesign themselves to better handle heat from like more parks and less freeways to more trees. Even just to opening cooling centers for people who don't have access to air conditioning.”