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The worst part of a heat wave is when it doesn’t cool off at night

A major heat wave is affecting most of the United States. Its biggest risks may come at night.

People cool off in a fountain in Washington Square Park on a hot afternoon in Manhattan on June 27, 2019, in New York City.
A heat wave is poised to broil the Midwest and the East coast this weekend. Some of the biggest health risks may come at night.
Spencer Platt/Getty Images
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

The National Weather Service is warning of a searing heat wave across the Midwest, South, and East Coast Friday and through the weekend. Heat advisories, warnings, and watches are in effect for nearly 200 million people in some of the most densely populated parts of the country.

And it’s not just the heat; humidity over much of the country will drive the heat index into triple digits. New York City is expected to reach a 110 degree Fahrenheit heat index on Saturday. AccuWeather projects that weather in Washington, DC, will feel about as hot as Death Valley.

New York City mayor Bill de Blasio declared a heat emergency, ordering city government and large buildings to limit energy use to “reduce strain on the electrical grid.”

Already this summer, record-breaking heat waves have baked India and Europe, and all-time heat records were broken in Alaska. Unusually hot weather has also cooked the Arctic, contributing to wildfires in Greenland north of the Arctic circle.

Forecasters Friday morning warned that the latest heat wave will be “prolonged, dangerous, and potentially deadly.”

And even after the sun sets, the high temperatures will still pose health risks: The worst health consequences of extreme heat often occur from hot weather at night.

High nighttime temperatures are behind some of the biggest health impacts of heat waves

Extreme heat is one of the deadliest weather phenomena in the world. There are direct health effects like heat stroke, which occurs when body temperature rises to 104 degrees Fahrenheit, leading to organ failure, and heat exhaustion.

But high temperatures can also worsen conditions like high blood pressure and can limit the effectiveness of certain medications. Heat can also exacerbate air pollution, which in turn can send people to the emergency room due to breathing problems.

Otherwise healthy people can also face health risks from extreme heat. In particular, outdoor workers like farm laborers, construction crews, and delivery personnel who have to spend hours in the heat stand to suffer. During last year’s heat wave, multiple workers died due to extreme heat on the job.

While it may cool off after the sun sets during a heat wave, it may not cool off enough for people who have been exposed to high temperatures all day. That leads to a higher cumulative exposure to heat.

One study examining the 2003 heat wave in Europe that killed upward of 70,000 people found that nighttime temperatures were a key indicator of the health risk from high temperatures. There’s also research that shows high nighttime temperatures disrupt sleep. Without relief from the heat, the stresses on the body mount.

Over the weekend, forecasters expect evening temperatures will stay above 80 degrees Fahrenheit, with the heat index remaining above 90 degrees, in some areas along the East Coast. That will make it hard for some to cool off. Health officials advise staying hydrated, wearing light clothing, and avoiding the outdoors.

Part of the reason temperatures stay high after sunset in many parts of the country is because of the urban heat island effect. Dense cities with their concrete, steel, glass, and asphalt soak up more heat than their rural surroundings, causing temperatures to rise further than they would have otherwise during the day. In the evening, those artificial surfaces continue to dissipate their accumulated heat, keeping denizens from keeping cool.

Our efforts to keep cool can also paradoxically make cities heat up. Air conditioners venting hot air outside can contribute to urban warming, and if the electricity that powers them comes from fossil fuels, they can increase the amount of heat-trapping gases in the atmosphere.

Another factor to consider: As the climate changes, summer nights are warming faster than days. “In general, since records began in 1895, summer overnight low temperatures are warming at a rate nearly twice as fast as afternoon high temperatures for the U.S. and the 10 warmest summer minimum temperatures have all occurred since 2002,” according to the National Oceanic and Atmospheric Administration.