Notable DC Heat Waves

Weather records date back to the 1870s in the Nation’s Capital and show there have been many memorable heat waves.  However, as the DC Metro Area has gotten more developed and urbanization has increased, recent heat waves have generally featured warmer overnight low temperatures compared to decades ago.  That can largely be attributed to a more pronounced “urban heat island” effect.

The Nation’s Capital has had a total of 88 record high-low temperatures over the last two decades between June 1 and August 31.  That includes records that were set and then broken again.  A record “high-low” temperature means the coolest temperature of the day was much warmer than average.  Meanwhile, there have been only six record low-high temperatures during the same three-month period in the last 20 years.  A record “low-high” temperature means high temperatures were much cooler than average on a given day. 

In addition, Washington, D.C. has had a total of 32 record high temperatures in June, July and August over the last 20 years.  That’s compared to only one record-tying low temperature during that time, (58° at National Airport on August 7, 2004).  Not since 1979 has the Nation’s Capital had a record low temperature during the month of July.  Meanwhile, the last time a record low occurred in the Nation’s Capital during the month of June was in 1992.

Although record warmth has occurred much more frequently than record cool weather in recent decades, triple-digit heat typically occurs only every few years.  Prior to the summers of 2010-2012 that each featured many 100° days, DC hadn’t had any since 2007.  DC residents didn’t experience 100° heat again until 2016.  There has been none since then.   Overall, Washingtonians have experienced triple-digit heat in eight of the last 20 summers. However, triple-digit heat on consecutive days occurs infrequently in the Nation’s Capital, having happened in only five summers over the last two decades.

Triple-digit heat isn’t required in a “heat wave,” which is defined as a minimum of three consecutive days of 90°/+ heat.  DC’s longest heat wave in the last 20 years was an 18-day stretch in 1999 (July 22 – August 8).  More recently, a 16-day heat wave occurred from July 18 – August 2, 2011.  Washingtonians also experienced 13-day heat waves in 2016 and 2006; both featured at least one day of triple digit heat.  No record-challenging heat is expected in the DC Metro Region for the rest of August.