Triple-Digit Heat in Washington, D.C.

Triple-digit heat is uncommon in the DC Metro Area, unlike parts of the desert southwest where it occurs almost daily during the summer months.  The eastern United States is typically much more humid than the arid climates of Arizona and nearby states.  Dry air heats up and cools off more quickly than more humid air, so areas where it’s less humid can see a higher difference between daily high and low temperatures.  

Phoenix, Arizona has an average daily high temperature of at least 100° from June through mid-September according to NOAA, while DC’s hottest average daily high temperature is 89° in mid-July.  The hottest temperature on record in Phoenix is 122° that occurred on June 26, 1990.  Meanwhile, DC’s hottest observed temperature is 106° that occurred twice, most recently on July 20, 1930.  

Another key difference is the frequency of triple-digit heat.  While essentially a daily occurrence in cities like Phoenix or Tucson during the summer, Washington, D.C. only sees triple-digit heat every few years.  There have been only six months in the last 25 years that have had three or more days of 100° heat in the Nation’s Capital.  Having three or more consecutive 100° days is even rarer in Washington, D.C. with only three such occurrences since 1993.  The most recent time Washingtonians experienced 100° heat on three consecutive days was August 13 – 15, 2016.

What makes the Nation’s Capital so uncomfortable during the summer months is the characteristic very humid weather.  The hazy, hot and humid conditions so pervasive over a large portion of the eastern United States can be attributed to the Bermuda High.  The “Bermuda High” is a large, semi-permanent feature in the subtropical Atlantic Ocean that’s often centered near the island of Bermuda.

 

Areas of high pressure have clockwise wind flow around them in the Northern Hemisphere.  Consequently, the southerly winds on the west side of the Bermuda High help pump warm and tropical air from the Gulf of Mexico northward toward major East Coast cities.  As a result, Atlanta, Washington, D.C. and New York City often experience very hot and humid weather during the summer.

The frequently oppressive summertime relative humidity in many eastern cities is both a good and a bad thing.  While it feels more uncomfortable with a higher “heat index” temperature (the feels like temperature when you combine the air temperature with the relative humidity), the actual air temperature isn’t as hot.

While some tout the “dry heat” of the southwestern United States, the more humid weather in the DC Metro Area during the summer helps create the potential for more rainfall.  Washington, D.C. averages 10.44” of rain from June 1 through August 31, while Phoenix averages roughly 20% of that total over the same period.