DC’s Hottest Month Gets Underway

Washingtonians typically experience the hottest weather of the summer during July.  Average high/low temperatures range from 88°/70° on July 1 to 88°/71° on July 31.  In between, however, are DC’s warmest annual average high/low temperatures of 89°/71° from July 7 through July 22. 

Thirteen of the last 20 July’s have been warmer than average in the Nation’s Capital.  Four of DC’s 10 hottest July’s have occurred since 2010.  DC’s hottest observed July temperature was 106° on July 20, 1930 and August 6, 1918. 

It’s worth pointing out that weather records were measured downtown, and away from the moderating influences of the Potomac River, before National Airport opened in the early 1940s.  DC weather records have been measured there ever since.  DC’s coolest July temperature is 52° which has occurred more than once, most recently on July 4, 1933. 

Since 1999, there have been a total of a 44 record high and record high-low temperatures during the month of July in the Nation’s Capital.  A record “high-low” temperature means a daily low temperature remained unusually warm.  Over the same period of time, DC residents have experienced only three record low-high temperatures, meaning three days had significantly cooler than average high temperatures.  DC hasn’t had a record low temperature during the month of July since 1988.  Part of this can be attributed to the proximity of National Airport to the Potomac River because it’s more difficult for areas on or near the water to heat up and cool down that areas away from bodies of water.

The Nation’s Capital has also seen a more pronounced urban heat island effect in recent decades.  That’s why temperatures away from National Airport are often much different, especially in the winter.  The added pollution caused by increased population and development is also detrimental to the environment.

Eleven of the last 20 July’s have been wetter than average in Washington, D.C.  The last two July’s have had 9.15” and 9.73” of rain, respectively, in the Nation’s Capital. That’s the only time DC has had more than 9” of rain in consecutive July’s in the more than 140 years of record keeping.  That’s nearly triple DC’s July average of 3.73”.  

The two very rainy July’s of 2017-2018 were memorable as they occurred during very different stretches of weather in DC Metro Area.  The period of August 2015 through March 2018 was a very dry period in the Nation’s Capital with an overall rainfall deficit of 16.42”.  On the other hand, April 2018 through December 2018 was an exceptionally rainy period with a rainfall surplus of 27.8”, and 2018 became DC’s wettest on record.  

NOAA expects a 33% chance for July 2019 to be a warmer than average month in Washington, D.C., with an equal chance for above or below average rainfall.

Washington, D.C.’s Hottest July’s (Source: NOAA)

1.  84.5° (2011)
2.  84.0° (2012)
3.  83.1° (2010, 1993)
5.  83.0° (1999)
6.  82.7° (2016)
7.  82.6° (1987)
8.  82.3° (1980)
9.  82.1° (1955)
10.  81.9° (1988)