DC’s Hottest Time of the Year

Heat Advisories In Effect Today  (Source: National Weather Service)

Washingtonians often experience some of the hottest weather of the summer during mid-to-late July.  That’s when average daily high temperatures are 89°.  The weather pattern is often such that hazy, hot and humid conditions dominate local weather headlines.  July 2020 is no different with some of the hottest weather of the year on tap the next few days. 

High temperatures will approach the mid-to-upper 90s today through Tuesday.  Some parts of the DC Metro Area will more than likely touch 100°.  However, these temperatures aren’t record-setting as DC’s record highs are all above 100°.  Nevertheless, it will still be dangerously hot.  The combination of the high temperatures coupled with moderate to high relative humidity will produce heat indices of 100° to 110°.  That’s led the National Weather Service to issue heat advisories for the entire DC Metro Area.

Yesterday was DC’s hottest day of the year with a high temperature of 98°.  It hasn’t been 100° since August 15, 2016.  Since the Nation’s Capital averages one to two days of triple-digit heat every four to five years, DC is due for some triple-digit heat.  Whether or not the actual air temperature reaches 100°, it’s important to limit time outdoors during such heat.  Drinking extra water, doing outdoor activities early in the day when its cooler, and applying suntan lotion are all good safety tips. 

Since 1997, there have been only nine summers in the Nation’s Capital with triple-digit heat.  Six of those summers featured three or more days of 100° heat, including four between 2010 and 2016.  One of the factors making triple-digit heat uncommon in the Nation’s Capital is where weather measurements are made.  Although DC weather records date back to 1871, they have been taken at National Airport since World War II.  A southerly wind at National Airport blows off the Potomac River and that often keeps the air temperature a few degrees cooler there during summer than downtown areas away from the river.

The Nation’s Capital recently experienced its third longest heat wave of 20 days, ending on July 15.  A “heat wave” is defined as a minimum of three consecutive days with highs of at least 90°.  DC’s average high during that 20-day stretch was just under 92°.  Since DC’s average high is 89° for much of July, temperatures in the low 90s aren’t unusual.  What made that heat wave noteworthy was its duration, not its intensity.  The current heat wave will be different as high temperatures will climb into the mid and upper 90s for much of the week, with 100s for some parts of the DC Metro Area.

July 19 Record Highs (Source: National Weather Service)

Washington, D.C., National Airport (DCA): 102° (1930)
Dulles Airport, Sterling, Virginia (IAD): 98° (1977)
Baltimore, MD, BWI Airport: 103° (1930)

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July 20

DCA: 106° (1930)
IAD: 101° (1980)
BWI: 102° (1930)

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July 21

DCA: 104° (1926)
IAD: 101° (1991)
BWI: 104° (1930)