Brutal heatwave extends 100-day series of 38 degrees Celsius

By Andrea Thompson

Over the past 100 days, temperatures in Phoenix, Arizona, have soared to 100 degrees Fahrenheit and often well above that. In that city and much of the heat-stricken Southwest, that streak will continue for at least a few more days as another heat dome settles over the area, driving temperatures higher.

The heat wave punctuates what has been a record-breaking summer in parts of the Southwest, including the metropolitan areas of Phoenix and Las Vegas — a summer that has surpassed previous records for the hottest meteorological summer set in those cities just last year. “It’s pretty unusual for something like this to happen two years in a row,” says Matt Salerno, a meteorologist with the Phoenix office of the National Weather Service (NWS). “It’s quite an accomplishment.” Even more astonishing, the city has beaten its record from last year by nearly two degrees Fahrenheit (one degree Celsius), reaching an average temperature of 98.9 degrees Fahrenheit (37.2 degrees Celsius). “That’s incredible when you just think about the average temperature we live in here in Phoenix,” Salerno says.

Las Vegas also set records in “pretty much every way,” says Matt Woods, a meteorologist in the NWS’s Las Vegas office. “It was brutal.”

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In both cities, the heat this summer has been a bit different than last summer. In the Southwest, June started with mild temperatures last year, followed by a July that exceeded expectations, and things cooled off again in late August as Hurricane Hilary roared through. While this summer hasn’t hit the same highs as July 2023, it’s been consistently sweltering from the start. In Las Vegas, “we’ve only had six days this summer with highs below 100 degrees Fahrenheit,” Woods says. Meanwhile, Phoenix’s streak of 100-degree F days has far surpassed the city’s previous record of 76 consecutive days, set in 1993.

The record-breaking heat of both summers is due to a series of heat waves, in which hot air masses linger over an area for days. “That’s basically what we’ve had for most of the summer,” Woods says. The clear skies that accompany these events mean that “the land surfaces are warming day after day,” which in turn warms the air directly above the surface, Salerno says. Such heat waves occur more frequently, last longer and become more intense because the excess heat is trapped by greenhouse gases produced by the burning of fossil fuels.

This week’s heat wave won’t be as intense as the one that smothered the region in July, and it won’t last as long, “but it’s definitely unusual for this time of year,” Woods says. It extends the time residents are exposed to conditions that pose a serious risk of illness. “Just because temperatures aren’t as high as they were in July doesn’t mean they’re not dangerous,” Woods says. In the U.S., extreme heat is more deadly than hurricanes, floods and tornadoes combined.

Those most at risk include very young children, the elderly, people with certain pre-existing medical conditions or taking certain medications, people who work outdoors, and homeless people. Both Woods and Salerno stress that everyone should stay hydrated during any heatwave, avoid going outside during the hottest part of the day, and seek shade or an air-conditioned environment when possible. Both scientists also warn against leaving a child or pet unattended in a car. So far this year, 27 children have died of heatstroke after being left in a car.

“Basically, all heat-related deaths are preventable,” says Kristie Ebi, an epidemiologist who specializes in heat-related health risks. Many cities, including Phoenix, are looking for ways to better warn residents and reduce deaths and hospitalizations from heat-related illnesses. The NWS is also working on better warning methods, including a new experimental tool called HeatRisk that shows places where the heat will be extreme enough to pose a serious health risk.

Although the heat of the day can pose a clear and obvious risk, both Salerno and Woods stress that warm nights are an underestimated threat because they don’t give the body enough time to cool down. “Everyone likes to look at the high temperatures,” says Woods, but “in reality, it’s the low temperatures that are the silent killer.”

The average low temperature in Phoenix this summer was 87.5 degrees Fahrenheit (30.8 degrees Celsius) — better than last summer’s average low of 84.8 degrees Fahrenheit (29.3 degrees Celsius). This year’s season saw 37 days where the nighttime low was at or above 90 degrees Fahrenheit (32.2 degrees Celsius), Salerno says. “Our lows are the highs of most places,” he adds.

And now September is adding to that danger and misery, already “far above what we would expect for this time of year,” Salerno says. Temperatures in Phoenix and the surrounding area could reach over 110 degrees Fahrenheit, and it’s likely that this event will see the city surpass last year’s record, when there were a total of 55 days with highs of 110 degrees Fahrenheit or higher.

It’s possible the heat will even linger into next week. Last year, the last 109-degree day in Phoenix was Sept. 10, 2023. (The latest 109-degree day in the city was Sept. 19, 2010.) “I hope that’s it for the 109-degree heat,” says a tired-sounding Salerno.