The sign of the fall season in Washington. (George Jiang via Flickr )

After much anticipation, crisp autumnal air arrived in the nation’s capital over the weekend. In its wake, temperatures tumbled, with D.C.’s temperature finally dropping below 60 degrees for the first time since early June.

At Reagan National Airport, the morning low dipped to 59 degrees on Sunday (Sep. 25), ending a 3½-month run during which temperatures never once once fell below 60 degrees. Before Sunday, D.C. hadn’t seen a temperature below 60 since Jun. 9, when the morning low was a delightful 53 degrees.


Only D.C.’s record-warm 2010 summer cooked up a longer stretch of temperatures at or above 60 degrees, spanning 112 days from May 22 through Sep. 10.

Embedded in this year’s 107-day run above 60 degrees, D.C. also recorded the longest stretch of temperatures at or above 70 degrees (35 days), the longest stretch at or above 65 degrees (83 days), and the second longest stretch at or above 80 degrees. The 65-degree streak obliterated the previous record — set in 2012 — by 9 days.


(ThreadEx)

For perspective, D.C.’s longest streak of 60-degree temperatures averages about 77 days annually, based on 1981-2010 normals. So if summer felt a month longer than normal this year, it wasn’t your imagination.

Such long periods of elevated temperatures are relatively new to D.C.’s historical climate record. 2016 saw not only the second-longest stretch of temperatures 60 degrees or higher, but it was also the fourth straight year of the streak lasting more than 100 days. Before 2014, 60-degree streaks lasting 100 days or more had never occurred in back-to-back years.

If we plot the longest stretch of low temperatures at or above 60 degrees since D.C. weather records began, you will notice a pronounced upward trend:


(Justin Grieser)

Recent streaks over 100 days are a far cry from D.C.’s shortest 60-degree streak, which was just 13 days in 1910. At National Airport, D.C.’s official weather station since 1945, the shortest such period was 27 days in 1946. More recently, 2004 saw 46 consecutive days at or above 60 degrees, marking the 10th-shortest period on record there.

Analysis

While many of us consider downtown lows in the 50s the exclusive domain of autumn, it didn’t used to be that way. As recently as the 1990s, D.C. would typically see an August day with low temperatures in the refreshingly cool 50s every two or three years. Now, such days are becoming a distant memory, as temperatures below 60 are hard to come by even in the first half of September.


(Justin Grieser)

The average date of D.C.’s first overnight low below 60 degrees is Sep. 2 (based on 1981-2010 climate normals). At National Airport, the historical average, including 2016, is Aug. 30. And if we consider D.C.’s entire span of weather records since 1872, the first day to fall below 60 degrees would typically occur around Aug. 21.

(One caveat: Despite 2016 having the second-longest streak at or above 60 degrees, it’s not the latest first occurrence of 50-degree temperatures in September. During the record-warm September of 1881, D.C.’s temperature fell to 60 degrees four times but never once dipped into the 50s. Back then, however, D.C. managed to drop below 60 in August. Furthermore, the longest stretch at or above 60 degrees in 1881 was a mere 54 days — about half what we just saw in 2016.)

Even if we dismiss the early part of D.C.’s climate record (when temperature measurements were recorded downtown instead of the current airport location), the long-term trend still shows temperatures in the 50s disappearing earlier in the summer and arriving later in the fall:


(Justin Grieser)

All told, the data show a remarkable rise in the frequency and persistence of mild overnight temperatures. Some of this increase can be attributed to increased urbanization in recent decades, as well as the relocation of D.C.’s observing station from downtown to National Airport. However, climate warming caused by rising global concentrations of greenhouse gases is undoubtedly amplifying the long-term temperature trend.

After our third-hottest summer on record, the return of temperatures in the 50s is a welcome change for downtown residents who want to give their air conditioners a full-time break. While D.C.’s cooler suburbs have seen a handful of days below 60 in recent weeks, this week marks the first substantial cool-down inside the Beltway (author’s note: I personally prefer overnight lows around 60 or lower before giving my A/C a rest).

Hopefully next year we won’t have to wait as long for seasonable fall weather to arrive. That said, a longer-than-average run above 60 degrees is probably a pretty safe bet.

Related:

D.C.’s temperature did not fall below 70 degrees for a record 35 days

Hot nights have increased a startling rate in Washington, D.C.

Data show marked increase in D.C. summer temperatures over past 50 years

Warm days and nights are near-record levels in D.C. and across the Northeast