A Single Number Can’t Capture Extreme Heat

Summer has only just officially begun, and the world is already sweltering. This week, two counties in northwest Maine were under their first-ever excessive-heat warning—part of a record-breaking “heat dome” that has settled on the eastern part of the country. Washington, D.C., might hit its first triple-digit high since 2016. Globally, the temperatures this spring have been even more shocking. Last week, the Sonoran Desert hit 125 degrees, the highest recorded temperature in Mexican history. Mecca, in Saudi Arabia, hit that same temperature. Last month, part of New Delhi, India, soared above 120 degrees.

These triple-digit highs are alarming and dangerous. In Mecca, hundreds of pilgrims making the Hajj pilgrimage to the holy Islamic city reportedly died in the heat. But in isolation, such temperatures can also be misleading. Not all 100-degree days are the same. The highest daily temperature isn’t the most revealing number about what a heat wave actually feels like, or what it does to our bodies.

One of the most destructive parts of a heat wave is not the highest highs, but the lowest lows. In other words, what’s worrisome isn’t just the daytime peaks but also the nighttime troughs. That’s because our bodies need the chance to cool off at night, recovering from the exertion of trying to keep us cool during hot days. But when temperatures stay high with no meaningful nighttime respite, that’s when the trouble really starts, Lisa Patel, a pediatrician and the executive director of the Medical Society Consortium on Climate and Health, told me.

Your heart pumps blood to the periphery of your body to cool down, eventually circulating it back to our hearts. But if it’s too hot at night, your heart just beats faster and faster trying to keep up, without a break before resuming such strenuous effort the next day. “These hearts just tire out, essentially,” Patel said. What counts as “too hot” is hard to say. One study found that deaths increased by 10 percent when temperatures stayed above 77 degrees Fahrenheit at night. The National Weather Service considers nights with a heat index (heat plus humidity) above 75 degrees, coupled with two days of highs at or above 105 degrees, as worthy of an “excessive heat warning” categorization.

Lingering nighttime heat is primarily a problem for people without air-conditioning or other ways to cool themselves off. If it’s 90 degrees outside but you’re nestled under a comforter with the AC set to 65, you’re likely just fine. Not everyone has that luxury, of course—and slightly cooler temperatures at night might spur penny-pinchers to turn off the AC.

Warmer nights are especially dangerous for older people, people with chronic conditions or heart disease, pregnant people, and children, who all have trouble regulating their body temperature. Newborns are particularly vulnerable: A study from the Catalonia region of Spain found that infants in the first week of life are particularly vulnerable to heat waves. City dwellers may also be especially at risk. They see hotter temperatures all throughout the day and night, as concrete and other urban structures absorb and re-emit more heat than natural surroundings. “It doesn’t get any better at night,” a Delhi rickshaw driver told CNN. On Tuesday, the city reported a nighttime temperature of 95 degrees, its hottest in more than a decade.

Besides nighttime temperatures, there’s another indicator to keep in mind: humidity. It plays a key role in how oppressive hot temperatures really feel, and how risky they are to our health. Humidity’s hazards come from how it can eliminate the effectiveness of sweating. If the air is already full of moisture, there’s nowhere for our sweat to evaporate. “Sweating is basically a sprinkler system,” Patel said. “Humid heat is much more dangerous to us because it takes away the one built-in mechanism we have to cool down.”

You might already be looking at temperatures that factor in humidity without knowing it. Heat combined with humidity creates a heat index, often referred to as the “real-feel” temperature on weather forecasts. But an even better measure of heat and humidity is something called the “WetBulb Globe Temperature,” or WBGT, which combines measurements of temperature, relative humidity, wind speed, and sunlight, using readings from three thermometers. Confusingly, one of those measurements is what’s called a “wet bulb” temperature—a separate indicator than the WBGT.

Unlike traditional temperature readings, which are calculated in the shade, the WBGT more accurately represents what it might feel like to be working outside in exposed conditions during a heat wave, and better accounts for how well sweat can evaporate. The U.S. military postpones non-essential physical activity at a WBGT reading of 90 degrees Fahrenheit. As of Friday midday, large swaths of the Midwest and mid-Atlantic regions had WBGT readings in the mid-80s, which the National Weather Service considers high to extreme. Air temperatures, meanwhile, were in the mid-90s. (WGBT is generally, but not always, lower than the regular temperature.)

Of course, that’s not all to say that dry heat in the middle of the day can’t be dangerous, too. Dehydration can occur as quickly as half an hour in high temperatures. But higher wet-bulb temperatures or higher nighttime temperatures can make hot days even more dangerous. If at night it’s hot and humid, your body is under even more strain just trying to maintain a healthy temperature.

If people want to be best prepared for heat waves, the top-line temperature on the weather forecast doesn’t cut it. Heat is already the leading weather-related killer in the U.S., responsible for more deaths than hurricanes, floods, and tornadoes. And temperatures are getting hotter. Overnight lows in the U.S. are rising twice as fast as daytime highs, according to a 2022 Climate Central analysis. The problem isn’t just what numbers we check, but how heat policies account for them. Phoenix—America’s hottest and fastest-growing city—extended hours for some cooling shelters this year. But only two will be open overnight. People don’t need to die during heat waves. Ensuring they don’t will be an easier task if heat is measured in the right way.