Some Perspective on DC’s Dangerous Heat

Dangerous July Heat in the DC Metro Area  (Source: WUSA9)

Starting today temperatures will approach the 100° mark for three consecutive days in the Nation’s Capital.  Having one day of triple-digit heat is relatively rare in Washington, D.C., let alone on consecutive days.  Even if high temperatures remain in the upper 90s, the relative humidity will combine with the above average temperatures to make it feel dangerously hot.

The “heat index” is what people feel when air temperatures are combined with relative humidity.  It will be between 105° and 115° each afternoon through Sunday and that’s as dangerous as it gets.  The last time any triple-digit heat occurred in Washington, D.C. was in August 2016.  That was also the last time DC residents experienced triple digit heat on consecutive days (August 13 – 15).

DC’s hottest temperature in all of 2018 was 98° on July 16.  Meanwhile, so far this year the hottest observed temperature in the Nation’s Capital was 96° on June 29.  Temperatures are poised to exceed that on each of the next three days.  It’s also worth noting that having three consecutive days of high temperatures ranging from 95° to 99° is also rare in the Nation’s Capital.  The last time that occurred was July 19 – 22, 2017. 

Although weather records in Washington, D.C. date back to the early 1870s, they have been kept in two different locations.  Since National Airport opened during World War II, weather measurements have been made there.  Prior to that, they were made downtown.  This is significant since National Airport’s location on the Potomac River has a moderating influence on temperature.  For example, if a south wind is blowing at National then the wind is blowing off the water.  During the summer that keeps temperatures slightly cooler than areas away from the River.

It’s also interesting to note that DC’s hottest overall temperature of 106° occurred twice prior to National Airport opening: once on July 20, 1930 and again on August 6, 1918.  Not only was that a downtown temperature, but Washington, D.C. was much less developed then.  There wasn’t as much of an urban heat island effect at the time, making those days of triple-digit heat even more impressive.  Today, overnight low temperatures don’t fall as much as they used too.  That’s the result of a much stronger urban heat island effect as the Nation’s Capital has become more developed combined with the fact that National Airport is located on the river.

That said it will be interesting to observe how high temperatures go the next three days.  Regardless of how hot it gets and whether or not any records are set, this weekend’s weather will be dangerously hot. 

Record High / High Low Temperatures (Source: National Weather Service)

July 19:

Washington, D.C., National Airport (DCA): 102° (1930) / 81° (2013)
Dulles Airport (IAD), Sterling Virginia: 98° (1977) / 77° (2013)
Baltimore, MD (BWI Airport): 103° (1930) / 80° (1942)

July 20:

DCA: 106° (1930) – DC’s hottest July temperature on record / 82° (2015)
IAD: 101° (1980) / 75° (2015 – has also occurred in previous years)
BWI: 102° (1930) / 80° (1930)


July 21:

DCA: 104° (1926) / 82° (1987)
IAD: 101° (1991) / 77° (1987)
BWI: 104° (1930) / 80° (1930) – BWI’s hottest July “low” temperature