Nights with lows of 80 degrees or higher were once rare in D.C. But in a warming climate, they’re becoming increasingly common.
Such warm nights can be especially dangerous because they offer little relief from daytime heat. During extreme heat events, the lack of overnight cooling can allow heat stress to build — particularly for vulnerable people without access to air conditioning — increasing the risk of heat-related illness or even death.
Below we take a look at the troubling trends in these unusually warm nights.
Annual 80-degree lows
Using climate averages from 1991 to 2020, Washington sees about two nights per year with lows of 80 degrees or higher, but the numbers can vary widely. There were none from 2020 through 2023, followed by a record eight in 2024, and then none again in 2025.
Over the long term, however, these unusually warm nights are becoming more common. From the 1870s through 2000, Washington recorded just 26 mornings with lows of 80 degrees or higher. Since 2000, there have been 47.
Several factors are likely driving the increase, including a warming climate and D.C.’s urban heat island, which tends to trap warmth overnight and limit how much temperatures can fall.
Longest streaks of 80 degrees or higher by year
Lows of 80 degrees or higher usually require a specific set of conditions, which is why they often occur in clusters. One factor is a very warm Potomac River. When the river reaches unusually high temperatures, it can help keep nighttime readings at Reagan National Airport, D.C.'s official observing site, from dropping very much. Similar effects occur downtown, where the urban heat island also limits overnight cooling.
The longest stretch of such nights on record lasted five days, from July 16 to 20, 2013.
When 80-degree lows are most common
All mornings with lows of 80 degrees or higher have occurred between June 16 and Aug. 17. The earliest came on June 16, 1981, and the latest on Aug. 17, 1995.
Most of these unusually warm nights occur in July, particularly during the second half of the month. In fact, 35 instances have been recorded between July 16 and July 31.
How hot days with lows in the 80s get
Heat tends to build on itself.
When the temperature fails to drop below 80 overnight, a hot afternoon is almost inevitable. Even the coolest days following such nights have reached 92 degrees, while the hottest have climbed to 105 degrees. The most common afternoon high on those days is 97 degrees.
Below are the warmest overnight lows on record. The highest is 84 degrees, which has occurred on three mornings — most recently in July 2011.
Taken together, these unusually warm nights highlight one of the clearer fingerprints of a warming climate. Temperatures in D.C. are rising fastest after sunset, when heat stored during the day lingers longer into the night. As the climate continues to warm, nights that once felt exceptionally muggy may become increasingly common.
Data from the Applied Climate Information System.
