Significant September Heat

Although DC’s average high/low temperatures both fall by 10° over the course of the month to 74°/57° on September 30, significant heat waves often occur in the Nation’s Capital.  A “heat wave” is defined as three or more consecutive days with high temperatures of at least 90°.  DC’s current heat wave began Monday, September 3 and will continue through tomorrow.  

A record-tying high temperature of 95° occurred yesterday in Washington, D.C.  That was the hottest temperature in the Nation’s Capital since July 16 (98°).  According to NOAA, today is DC’s eighth day in the 90s since August 27.  Making the current heat wave worse has been the very high relative humidity.  Humidity is “relative” to a given temperature since warmer air can hold more water vapor than cooler air.  That’s why higher relative humidity feels worse in the summer than it does in the winter.

Meteorologists look at the dewpoint temperature when calculating relative humidity.  The combination of air temperature and relative humidity produces the “heat index,” which is the feels-like temperature.  Some people consider the heat index to be summer’s equivalent to winter’s wind chill factor (i.e., the feels-like temperature in cold weather when you combine the temperature with the wind speed).  

The “dewpoint temperature” is the temperature at which 100% relative humidity or saturation occurs and dew forms.  If the dewpoint is below freezing, then frost develops.  Dewpoints in the 50s are comfortable, while dewpoints in the 70s are oppressive.  Dewpoint temperatures have been in the 70s for much of the last two weeks.  

The current heat wave is the most recent of memorable September heat waves that have occurred over the last decade in the Nation’s Capital.  Like this month, September 2008 got off to a hot start in Washington, D.C. with a four-day heat wave through September 5.  Also, September 2010 ranked among the ten warmest Septembers in Washington, D.C. and had two separate heat waves.  The longest occurred from September 22 – September 25, with September 24’s record high of 99°.  Washingtonians also experienced 90° on five of the first six days of September 2014.

Washingtonians also experienced two heat waves in September 2015, with the hottest temperature being 96° on September 3.  Finally, September 2016 tied with 1930 as DC’s third warmest September on record and had a total of seven 90° days.  Even rarer than a traditional heat wave in the Nation’s Capital is having three consecutive days of temperatures 95°/+.  That only occurred once in the last 30 years, from September 8 – 10, 2016.