About half of the U.S. population will have to contend with dangerous heat sometime this weekend as a blistering heat wave blankets much of the country. Record-breaking temperatures will reach well above 100 degrees Fahrenheit, with heat indexes that will feel like it’s 110 or higher.

But those record highs are not the biggest problem for cities.

As a heat dome begins its slow creep across the Midwest, many major U.S. cities in the Great Plains and Northeast will suffer through a weekend of uncomfortably hot weather thanks to high temperatures combined with excessive moisture in the air—some from the dissipating Hurricane Barry—creating humid, muggy conditions.

Thanks to those conditions, many of those cities will also see exceptionally high low temperatures, some as high as 80 degrees Fahrenheit. According to the National Weather Service, over 100 cities are expected to experience record-high overnight lows between now and Sunday.

Not only will afternoon temperatures and humidity be dangerously-high, but there won't be much relief at night for many.

RECORD warm low temperatures are expected too, with 123 records currently forecast to be tied or broken. pic.twitter.com/lnU5tC83ns — National Weather Service (@NWS) July 18, 2019

An 80-degree low is particularly dangerous because that’s the level at which the human body is unable to regulate its temperature. When people experience prolonged exposure to temperatures 80 degrees or higher without periods of relief, heat-related deaths are likely to occur.

Extreme heat now kills more people in the U.S. than all weather events combined. Infants, older adults, outdoor workers, people with preexisting medial conditions, and households without air conditioning are all at a greater risk.

These record high minimums can create emergency situations for vulnerable people living in cities. High overnight lows are more life-threatening in dense urban areas due to heat island effect, where roads and buildings can keep the air up to 15 degrees warmer than less developed areas, even as the sun goes down. And at night, public places like malls and other city-managed cooling centers are usually closed.

Relentless overnight lows were a major factor in the deaths of over 70,000 people in Europe during a 2003 heat wave that lasted three weeks. People may have taken precautionary measures to keep themselves cool during the day, but slept night after night in buildings without access to air conditioning, stressing their respiratory and cardiovascular systems.

Climate change is creating longer and more intense extreme heat events. The number of 100-degree days are expected to double in some parts of the South and Southwest by 2050, with deadly summer nights also becoming more frequent.

According to a new study by the Union of Concerned Scientists, cities must look at new ways to keep residents safe from “killer heat,” especially when a strained power grid is likely to produce blackouts and brownouts that could affect people’s ability to run air conditioners overnight.

When it comes to immediate relief, some cities, like New York City and Washington, D.C. are expanding the hours of public pools and beaches to provide additional access to cooling infrastructure even after the sun goes down. On a slightly longer timeline, planting more trees can help, too, especially if cities are putting them in areas where they can help cool places where people will be sleeping.

The heat wave arrives amid a torrent of bad news for global temperatures. The National Oceanic and Atmospheric Administration (NOAA) just certified June 2019 as the planet’s hottest June on record—and this month is already on track to be the warmest July.