Late August Heat

For the first time since April 30, DC residents will have a sunset this evening before 8:00 PM.  Despite the decreasing amount of daylight in Washington, D.C., blistering heat is a frequent occurrence during the second half of August and much of September.  In fact, today will be the second consecutive day of 90° heat with a third on tap tomorrow that will make the current stretch of hot weather an official “heat wave.”

DC’s average high/low temperatures today are 87°/70° according to the National Weather Service.  That’s down from DC’s hottest average daily temperatures of 89°/71° from July 7 – 22.  However, there isn’t much tangible difference between those subtle differences in average high/low temperatures. 

Last August, the Nation’s Capital experienced a five-day heat wave from August 27 – 31 and again from September 3 – 7.  DC’s most significant recent August heat wave occurred in 2016 with 12 consecutive days of high temperatures of at least 90° from August 10 – 21.  During that span was also the last time triple-digit heat occurred in the Nation’s Capital.  DC’s hottest and second hottest temperatures on record, 106° and 105°, respectively, have occurred in both July and August.  August is second only to July in terms of being the hottest month of the year in Washington, D.C.

DC’s hottest temperature on record, 106°, happened twice (August 6, 1918 and July 20, 1930).  Both instances occurred before National Airport opened in the early 1940s, where weather measurements have been ever since.  Prior to that, DC weather observations were made downtown away from the moderating influence of the Potomac River.  The Nation’s Capital was also much less developed than it is now, so the urban heat island effect wasn’t the issue it is nowadays.  

DC’s second hottest temperature has occurred three times, twice in July (July 7, 2012 and July 10, 1936) and once in August (August 17, 1997).  The latest in the year 100° heat was recorded in Washington, D.C. was on September 8, 1939 (100°).  An honorable mention goes to September 24, 2010, that featured a record high of 99°.  That was the 67^th and final 90° temperature of 2010 that tied it with 1980 for DC’s highest annual total of 90° days on record.

Triple-digit heat is a relatively rare occurrence in Washington, D.C., typically only happening every three to four years.  However, August temperatures of less than 60° have been even rarer in the Nation’s Capital in recent decades.  There have been only nine days with low temperatures in the 50s during August in the Nation’s Capital, dating back to the early 1990s.  By comparison, there have been 10 days of triple-digit heat in Washington, D.C. since 1993.  Although those totals are close, the key difference is the last time triple-digit heat occurred in DC during August was in 2016, while there have not been any <60° August temperatures since 2004.

August 19 Record Highs / Record High Lows (Source: NOAA)

Washington, D.C. (National Airport – DCA): 98° (2002) / 78° (2002)
Dulles Airport (IAD), Sterling, VA: 95° (2002) / 74° (2002)
Baltimore, MD (BWI Airport): 97° (1914) / 78° (1937)

August 20

DCA: 101° (1983) / 78° (2002)
IAD: 104° (1983 – hottest August temperature on record) / 74° (2002)
BWI: 105° (1983 – tied for hottest August temperature) / 78° (1914)