A closer look at DC’s hottest July’s

 

A summer's day in the nation's capital

Seven of the 10 hottest July’s in the nation’s capital have occurred over just the last 15 years.  That’s coincided with all-time high levels of urbanization and automobile traffic. Overnight low temperatures have been an important factor in calculating how warm a summer month is when compared to average.

The increased influence of the urban heat island effect helps overnight low temperatures remain warmer than they otherwise would be. The 10 warmest July’s in the nation’s capital have averaged roughly three nights when low temperatures remained 80 degrees or warmer. That’s exceptionally warm for the nation’s capital where the warmest average daily low temperature is 73 degrees.

Having warmer overnight low temperatures is the primary reason why so many of DC’s warmest July’s have occurred relatively recently. If days start warmer than they used to with higher daily low temperatures, then how warm it gets during the day is less important than it otherwise would be. For example, DC’s two hottest July’s occurred in 2011 and 2012, respectively. While July 2012 featured a record seven days of triple-digit heat, it finished 0.5 degree cooler than July 2011 that had only three days at or above 100 degrees. The reason for this is July 2011 finished with seven days low temperatures remained in the 80s compared to only four such days the following year.

Despite July 2012 having more triple-digit heat than July 2011, the average daily high temperature in the nation’s capital was still warmer in July 2011. That’s in large part due to DC’s record low-high temperature of 71 degrees on July 21, 2012. In other words, the high temperature in the nation’s capital on July 21, 2012, was cooler than what that average “low” temperature for the date should be.

Another illustration of the importance of warmer overnight low temperatures when calculating monthly average temperatures is the comparison of July 1930 and July 2020. The nation’s capital experienced six 100-degree days in July 1930, compared to none in July 2020. However, July 2020 finished 3.5 degrees warmer than July 1930. That’s largely due to the fact DC’s average low temperature for July 2020 is 75 degrees, compared to 69.3 degrees for July 1930. This further illustrates how significant the urban heat island effect is in urban and suburban areas combined with greenhouse gas emissions when looking at temperatures.