DC’s Hottest Month

 

July is the hottest month of the year in the Nation’s Capital.  Earlier this year, NOAA updated its 30-year climate data set to reflect the period of 1991-2020, replacing 1981-2010.  Consequently, DC’s average temperatures and rainfall are both higher than they were previously.  That shouldn’t surprise many, however, as the decade of the 2010s were considerably warmer and wetter than the 1980s.

DC’s average July temperature (combining daily high/low temperatures) for 1991-2020 was 81° compared to the 1981-2010 average of 79.8°.  Using the old statistic, four of the nine July’s between 2001 and 2009 would have been warmer than average in the Nation’s Capital, rather than being nine consecutive cooler than average July’s. 

Triple-digit heat has occurred in four July’s since 2010 and most recently in 2016.  Six of DC’s 10 hottest July’s have also occurred over the last eleven years.  July is such a hot month in Washington, D.C. that 28 of the 31 daily record highs during the month are at least 100°.  By comparison, that number drops to 19 in August.

July is also a much wetter month than it used to be based on NOAA’s updated climate statistics.  The Nation’s Capital now averages 4.33” of July rainfall, compared to 3.73” in the 1981-2010 data set.  Not only have seven of the last 10 July’s been wetter than average, but Washingtonians experienced two of the five wettest July’s on record in 2017 and 2018.  Those months helped boost July’s average monthly rainfall by 0.6” when NOAA updated its climate averages.

Washingtonians have gotten used to heavy rain and record heat during the month of July over the last decade.  Something D.C. residents haven’t seen much of in recent years has been record cool temperatures.  The last time a record low occurred in Washington, D.C. during July was in 1988.  The warmer temperatures and higher average July rainfall are not unrelated as warmer temperatures can hold more water vapor.  Not since 2016 have Washingtonians gone through the entire month of July without at least one daily rainfall total of at 2”/+. 

According to NOAA’s Climate Prediction Center, there is an above average probability for July 2021 to be a warmer and wetter than average month.  That would make this month DC’s sixth warmer than average July in the last seven years and eighth wetter than average July in the last nine years.