How Hot Can August Get?

Although average temperatures in August aren’t quite as hot as July in Washington, D.C., it can feature exceptionally hot weather.  Washingtonians have experienced triple digit heat during five Augusts over the last twenty years, compared to six different July’s – including last month.

Contributing to the warm summer weather in the eastern United States is the feature known as the “Bermuda High.”  A sprawling area of high pressure that sets up for most of the summer just off the East Coast – hence the name “Bermuda High” – this powerful area of high pressure acts as a heat pump ushering in warm and humid weather northward from the south.   One of the characteristics of an area of high pressure is sinking air, which also serves to suppress cloud formation.  That also allows the temperature to rise. 

The hottest temperature on record in Washington, D.C. is 106 degrees and has occurred twice (July 18, 1930 and August 6, 1918).  Meanwhile, the hottest temperature on record at National Airport (which opened in 1941) is 105 degrees and also occurred twice (August 17, 1997 and July 7, 2012).  However, there are rare instances where it can remain cloudy and cool during the summer months.  Such was the case on this date in 1996 when high temperatures reached only 75 degrees at both National and Dulles Airports.

Although Washington, D.C.’s average high temperatures are slightly warmer in July, there have been instances where August was hotter than July.  Such was the cause during the torrid summer of 1980 that saw the hottest August on record in the Nation’s Capital.  August’s average monthly temperature (combining daily highs and lows) is 78.1 degrees in Washington, D.C. compared to July’s average temperature of 79.8 degrees – based on NOAA’s 30-year average through 2010.  While August was warmer than July in Washington, D.C. seven times from 2000 through 2009, July has been warmer every year since.

A key difference in recent years has been the tendency for overnight temperatures to remain warmer in urban areas.  National Weather Service data indicates the Nation’s Capital just this week set a new record for longest stretch where temperatures remained above 70 degrees on 35 consecutive days from July 4 through August 8.  The previous record was 32 days from the record summer of 1980.  It’s interesting to note that three of the other top 10 streaks for highest number of consecutive days at or above 70 degrees in Washington, D.C. have occurred since 2010.  

The warmer overnights can be attributed to an increased amount of urban development and greenhouse gas emissions.  That creates a type of self-fulfilling prophecy since people will use more energy to keep cool and that will, in turn, lead to more greenhouse gas emissions.  Fortunately, average temperatures continue their gradual decline as Labor Day approaches.