A Closer Look at DC’s 2021 Summer Weather

 

Despite the hot weather the last few days, this month’s temperatures are poised to finish within 0.5° of average.  That’s in stark contrast to last July in the Nation’s Capital that was the third warmest on record.  This month will finish slightly warmer than average with a little less rainfall than average. That follows June, which finished a bit warmer than average with a rainfall surplus of 1.31”.

There were two days with more than an inch of rain in Washington, D.C. during the first nine days of July.  By comparison, there has been a combined rainfall total of less than an inch of rain in the Nation’s Capital since July 10.  Barring any heavy rain from a thunderstorm during the next few days, this month will finish as DC’s first drier than average July since 2016.  Two of DC’s five wettest July’s on record have occurred in the last four years (2017, 2018).

High temperatures the last two days reached the upper 90s in Washington, D.C. for the first time in more than a year.  However, there have been several days this month with highs in the low-to-mid 80s and comfortable relative humidity that’s broken up the stretches of high heat and humidity.  No day was more fall-like than July 3, with a high/low temperature of 81°/63°.  As of yesterday, there have been 17 days of 90° so far this month.  However, that’s relatively tame compared to last July that had a record 28 such days. 

Because weather measurements for Washington, D.C. are made at National Airport where the Potomac River has a moderating influence, triple-digit heat is a relatively rarity.  If there’s a south wind in the Nation’s Capital, then it blows off the cooler waters of the Potomac River at National Airport.  That tends to keep temperatures down slightly compared to downtown areas away from the river.  DC’s hottest days have a west or northwesterly wind and are a little less humid than the days with a southerly wind.  No triple-digit heat has occurred in Washington, D.C. since August 15, 2016.

Last month featured a similar story with temperatures.  Washingtonians had seven days in the 90s last month, but another six days high temperatures remained in the 70s.  While 5.51” of rain fell in the Nation’s Capital, 4.62” (or roughly 84%) fell during a five-day stretch.  That’s why temperature and precipitation extremes are often hidden when looking at monthly weather averages.