Is Triple-Digit Heat Rare in the Nation’s Capital ?

 

D.C. residents who don’t like extreme heat should be pleased that another June will pass without any triple-digit heat.  This June will finish as a warmer than average month in the Nation’s Capital.  Record heat occurred at D.C.’s three area airports on June 17, including a high of 99° at National Airport.  There have been four days of 90° heat so far this month.

However, it’s important to remember that the Nation’s Capital has averaged between seven and eight 90° June days dating back to the early 1990s.  It has also averaged one day of triple-digit June heat roughly once every five years over the same time period.  But, D.C. residents haven’t experienced 100°/+ heat in June since 2012.

Triple-digit heat is relatively rare during any month in the Nation’s Capital, typically occurring only every three to four years.  Washingtonians haven’t experienced triple-digit heat since August 2016.  It’s important to distinguish between the actual air temperature and feels-like temperature.  The “heat index” is what it feels like when you combine the air temperature with the relative humidity.  On very humid days, it feels hotter and more oppressive compared to less humid days.  The heat index exceeds the century mark several times over the course of a typical D.C. summer, but the actual air temperature rarely touches 100°.

D.C.’s hottest temperature on record is 106° and has occurred twice, most recently on July 20, 1930.  D.C.’s second hottest all-time temperature is 105° and has also occurred twice, on July 7, 2012 and August 17, 1997.  Although weather records in the Nation’s Capital date back to 1871, the more significant date to remember is 1941.  That’s when National Airport opened and where weather records have been kept ever since.  Prior to that, weather measurements were made downtown.  Conditions are quite different between the two locations because one has the moderating influence of the Potomac River and the other has more of an urban heat island effect.

There were some very hot summers in the 1930s prior to when National Airport opened.  However, a key difference between then and now is that urbanization has increased dramatically.  Consequently, overnight low temperatures remain much warmer than they used to.  That’s why the Nation’s Capital has had many of its warmest months on record over the last 10 to 20 years.  For example, June 2010 was D.C.’s warmest on record with the highest number of 90° days (18).  D.C.’s hottest temperature during all of last year was 97°.  This serves to underscore how rare triple-digit heat is in the Nation’s Capital.