The District at night in May 2017. (Nathan Jones via Flickr)

The heat wave currently affecting much of the United States isn’t going to set many daytime high-temperature records. Instead, the most notable aspect will be the oppressive humidity levels that will cause heat indexes to skyrocket and prevent overnight temperatures from dropping to seasonable levels.

For example, overnight lows in the District through the weekend are unlikely to drop below 80 degrees. Heat indexes may remain near 90 each night.

This characteristic of the current event is in keeping with longer-term trends, as summer low temperatures have been increasing far faster than daytime highs.


Trend in summer overnight low temperatures in the United States since 1895.

Deke Arndt, who leads the climate-monitoring branch at the National Centers for Environmental Information in Asheville, N.C., created some helpful graphics to illustrate how the likelihood of extremely warm nights has increased in recent decades compared with during the 20th century.

As Arndt noted in a Twitter thread and a subsequent interview with the Capital Weather Gang, hot overnight temperatures have significant consequences. For example, they prevent people without air conditioning from experiencing relief at night, which tends to increase public health risks.

“This isn’t a curiosity: it’s expensive and draining in certain seasons and places and economic sectors and populations,” Arndt tweeted. “It’s one of the subtleties in a changing climate that has massive economic and social consequences.”

The reason summer nights have been warming faster than summer days, Arndt says, has to do with the structure of the atmosphere at night. “The layer of air in contact with ground is smaller at night. In the night when the atmosphere is typically less active and doesn’t mix to the depths that it does during the day, that additional warming effect can be amplified at night,” he said in the interview.

Arndt created graphics for the District and other locations to show how the climate is changing the likelihood of hot nights.

The graphics show bell curves of the overnight-low-temperature distribution during two periods, the years from 1960 to 1989, and 1990 to 2018. In each case, the later curves shift to the right.


Probability distribution of summer nighttime temperatures in the District during two time periods. (Deke Arndt, NCEI).

“As you slide to a warmer climate, you’re sliding from left to right on those graphics,” Arndt says.

The middle of the bell curve shows the most common summertime low temperature. But what’s more interesting — and relevant, in the case of a heat wave — is what’s happening at the extreme ends of the distribution, also known as the “tails.”

“Part of that sliding motion means the really warm, very unusual kind of nights, those extremes tend to increase disproportionately when you move the whole thing over to the right,” Arndt said.

Take the District, for example. What was a top 5 percent summertime low temperature is now 2.3 times as frequent, according to Arndt’s analysis. The old top 5 percent threshold was 72.5 degrees, but that has risen to 74 degrees. And what was a top 1 percent summertime night, occurring about once every 100 years, is now 2.7 times as common.


Probability distribution of summer overnight low temperatures in Leon County, Fla., for two time periods. (Deke Arndt, NCEI).

Other locations have seen even bigger shifts in overnight low temperatures during the summer. Owing in part to warming Gulf of Mexico waters, Leon County, Fla., where Tallahassee is located, has experienced a more dramatic boost in summertime low temperatures.

And in New York’s Queens borough, where overnight lows are predicted to remain near 80 degrees through this weekend, such extreme heat at night is becoming more common as well.


Probability distribution of overnight low temperatures during the summer in Queens County, N.Y., during two time periods. (Deke Arndt, NCEI).