How cool has June been ?

 

A June day at the Avenel Pool in Potomac, Maryland

D.C. residents who don’t like extreme heat should be pleased that another June will pass without any triple-digit heat.  Not only that, but June 2023 has been an unusually cool month in the nation’s capital.  Only four of the first 24 days this month have been warmer than average.  That’s helped produce a monthly temperature 2.6° cooler than average. 

There have been more days with highs in the 70s (four) than days in the 90s (two) since June 1.  The nation’s capital hasn’t finished with fewer than five 90° June days since 2014.  That’s significant since the nation’s capital has averaged between seven and eight 90° June days over the last 30 years, according to NOAA data.  It has also averaged one day of triple-digit June heat roughly once every six years over the same time period.  However, 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 with none 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 only 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).