D.C. Heat Waves

Happy Independence Day !

A “heat wave” is defined as a minimum of three consecutive days with high temperatures of at least 90°.  Every once in a while, atmospheric conditions are such that Washington, D.C. experiences an extended heat wave with 90°/+ heat on 10 or more consecutive days.  In fact, there have been 13 heat waves in the Nation’s Capital over the last 25 years that have lasted at least 11 days. 

D.C.’s longest heat wave in that time was an 18-day heat wave that spanned from July 22 through August 8, 1999.  While temperatures only reached triple-digits on one day, there were eight days with high temperatures between 95° and 99°.  D.C.’s average high/low temperatures for this sultry 18-day stretch were 94.4°/73.3°.  That was D.C.’s longest heat wave since a 16-day stretch from July 3 to July 18, 1993, which included four days of triple-digit heat and had an average high/low temperature of 95.4°/75.0°.  The summer of 1994 had a 14-day and an 11-day heat wave.

Washington, D.C. also had three heat waves lasting 11 or more days between the years of 2000 and 2009.  The first occurred from August 10 to August 20, 2002 and included one day of triple-digit heat and had overall average high/low temperatures of 96.1°/75.4°.  That was D.C.’s most significant heat wave since 1999.  Next, there was a 13-day heat wave between July 27 and August 8, 2006.  The following year, Washingtonians experienced an 11-day heat wave from July 31 to August 10. 

Finally, residents endured another four heat waves of at least 11 days just since 2010.  The longest was a 16-day stretch from July 18 to August 2, 2011 that included four days of triple-digit heat.  Meanwhile, D.C.’s most recent heat wave of at least 11 days was a 12-day stretch from August 10 to August 21, 2016.   That included three consecutive days of triple digit heat from August 13 – 15.  It was only D.C.’s third time in the last 25 years that 100°/+ temperatures occurred on three or more consecutive days.

Although these heat waves appear similar, there are two subtle differences.  The average low temperatures have increased slightly, with low temperatures of 80°/+ more common during the more recent heat waves.  That’s largely the result of an increased amount of development and suburban sprawl that accentuates the urban heat island effect.  In addition, three of the four most recent extended heat waves (which have occurred since 2011) included three or more days of triple-digit heat.  Prior to 2011, the last D.C. heat wave to include three or more days of 100°/+ heat was in July 1993.