Days with highs of 90 degrees or higher are a staple of summertime in Washington. On average, they occur about 40 times a year — roughly one out of every three days between June and August.
Because of human-caused climate change, these hot days have become more common in recent decades. Still, there is considerable year-to-year variability, with some summers far steamier than others.
Below are some of the key statistics and trends behind D.C.’s 90-degree days.
Averages for 90-degree days
Based on 1991–2020 climate averages, the first 90-degree day in D.C. typically arrives around May 16, while the final one occurs near Sept. 12.
Highs in the 90s most often occur between May and September. On average, the monthly breakdown looks like this: two in May, seven in June, 15 in July, 10 in August and three in September.
The earliest 90-degree reading on record came unusually early — March 22, 1907 — while the latest occurred Oct. 11, 1919. In contrast, the latest first 90-degree day didn’t arrive until July 12, 1979, when the high reached 93 degrees.
90-degree days annually
D.C. has experienced as many as 67 days with highs of 90 degrees or higher in a single summer, which occurred twice — most recently during the record-warm summer of 2010 (average temperature 81.3 degrees) and earlier in 1980.
At the other extreme, some summers have seen very few such days. The lowest totals on record are seven, observed in 1886 and 1905. Even in more recent times, relatively mild summers have occurred — including 2004, when the city recorded just 11 days at or above 90 degrees.
Longest streaks at or above 90 degrees
The longest streak of 90-degree days in a typical year lasts about nine days. But some summers have produced much longer runs. The longest streaks on record all exceed two weeks, topped by 21 consecutive days in both 1980 and 1988.
Using three straight days of 90-degree heat as the definition of a “heat wave,” D.C. averages about five heat waves per summer. The most in a single season occurred in 1991, when the city recorded 11 separate heat waves.
When D.C.'s average daily peaks at 90
Using the 1991 to 2020 climate averages, D.C.’s average daily high temperature now peaks at 90 degrees between July 6 and 27. (Previously, using 1981 to 2010 climate average, the peak average high was 89.)
More broadly, the core season for 90-degree heat stretches from late June through mid-August. Between 1871 and 2025, July 16 has recorded the most 90-degree highs of any calendar date.
