The days are hot, but so are the nights. That’s a problem, experts say
The days are hot but so are – Across much of the United States, sweltering daytime temperatures have reached perilous levels, with a persistent heat dome lingering over the Midwest and Atlantic Coast regions. However, the alarming trend extends beyond daylight hours—nighttime temperatures are also remaining unusually elevated, intensifying the strain on human health, ecosystems, and energy systems. This phenomenon, compounded by climate change, is creating a new challenge for communities already grappling with extreme heat during the day.
Why the Overnight Heat is Especially Harmful
According to Ashley Ward, director of the Heat Policy Innovation Hub at Duke University’s Nicholas Institute, the prolonged warmth at night is a critical factor in escalating heat-related risks. “Evening temperatures persist at unusually high levels, with some areas failing to dip below 80 or 75 degrees,” she explains. “This lack of overnight cooling is concerning, as it prevents the body from recovering from the day’s heat.”
“Our bodies need it [cooling] to function properly, but so do plants, animals, and energy infrastructure,” Ward adds. “The cooling period is essential for resetting systems and mitigating the cumulative effects of heat.”
The National Weather Service projects that Thursday’s high temperatures in the Baltimore-D.C. metro area and southeastern Pennsylvania will hover between 95 and 105 degrees Fahrenheit, with heat indices pushing past 110. Meanwhile, overnight lows are expected to remain in the low 80s, creating a prolonged period of discomfort. This scenario is becoming increasingly common, as climate change alters the balance between day and night temperatures.
Climate Change Intensifies the Heatwave Pattern
Climate change is amplifying the frequency and intensity of heatwaves, but its effects are even more pronounced at night. The Fifth National Climate Assessment, released three years ago, highlights a concerning shift: nighttime temperatures are rising faster than daytime temperatures across the U.S. “The number of nights where temperatures stay above 70°F is increasing everywhere except the Northern Great Plains,” the report states. This trend means that the body’s ability to cool down during sleep is being compromised, heightening the risk of dehydration and heat-related illnesses.
“Each summer, we’re witnessing more heatwaves and hotter nights,” says Alex DaSilva, a meteorologist at Accuweather. “The pattern is clear—overnight lows are consistently warmer, especially during major heat events.”
The reason for this disparity lies in the changing climate. DaSilva notes that the atmosphere now retains more heat after sunset, creating a “hotter floor” for the following day’s temperatures. “This means the heatwave doesn’t just start from a cooler baseline; it builds on a warmer one,” he explains. The result is a feedback loop that makes heatwaves more severe and longer-lasting.
Humidity Adds to the Dangers
Exacerbating the situation is the rise in humidity, a byproduct of global warming. “Higher moisture levels in the air reduce the effectiveness of sweating, which is the body’s primary cooling mechanism,” says W. Larry Kenney, a physiology and kinesiology professor at Penn State University. “Even after a rain shower, the air can feel just as oppressive, sometimes worse.”
“It’s not just the temperature that matters—it’s the combination of heat and humidity,” Kenney tells NPR’s Morning Edition. “Sweat evaporates more slowly when the air is saturated, leaving the body less able to regulate its internal temperature.”
For individuals without access to air conditioning, the situation is particularly dire. The U.S. Energy Information Administration reports that 88% of households had air conditioning installed by 2020, but this coverage is uneven. A separate study by the Kaiser Family Foundation (now known as KFF) reveals that Asian, Black, and Hispanic households are disproportionately likely to lack cooling systems, as are lower-income families. “This disparity isn’t just about convenience—it’s a matter of survival,” Ward emphasizes.
Who is Most at Risk?
Older adults are especially vulnerable to the effects of extended heat. Kenney points out that age-related changes in thermoregulation make them more susceptible to heat exhaustion and heat stroke. Comorbidities like heart disease or diabetes further compound these risks, as the body struggles to adapt to extreme conditions.
“The body’s ability to sweat decreases with age, and conditions like diabetes can slow the circulatory system, making recovery harder,” Kenney explains. “These factors create a perfect storm of health challenges during prolonged heat events.”
While the full extent of the impact remains under study, the consensus among experts is clear: the heat is not just a temporary inconvenience. It’s a growing threat that demands immediate attention. “The current heatwave is a snapshot of what’s to come,” DaSilva says. “If we don’t address this trend, we’re setting the stage for more frequent and intense heat events.”
Adapting to the New Normal
For those who must remain outdoors, Kenney recommends gradual acclimatization. “It’s important to expose the body to heat slowly, allowing it to adjust without overextending its capacity,” he advises. Staying hydrated is also crucial. “Drink water before and after heat exposure to compensate for fluid loss,” he says. “This is especially vital for older individuals or those with chronic health conditions.”
Communities must also invest in infrastructure to support those without access to cooling. “Air conditioning is a lifeline for many during heatwaves, but it’s not universally available,” Ward states. “This inequality puts certain groups at higher risk, particularly in urban areas where heat islands amplify temperatures.”
As the heat dome continues to cast its shadow, the urgency to prepare is growing. With nighttime temperatures no longer providing relief, the cycle of heat is becoming more relentless. “This is a defining feature of our changing climate,” DaSilva says. “The days are hot, and the nights are just as dangerous—so we need to rethink how we respond to heat.”
