Fun Facts About DC Area Heat Waves

Heat waves are defined simply as any period lasting three or more consecutive days with highs of at least 90°.  However, it’s common to see 90° in July given that average high temperatures in the Nation’s Capital are 89° from July 7 – July 22.  DC’s high temperature of 91° yesterday was the 18^th consecutive day of the current heat wave that began on June 26.  That tied it with an 18-day heat wave that occurred in 1999 and was DC’s third longest.  Another 18-day heat wave occurred in the Nation’s Capital in 1872.  The record for DC’s longest heat wave is 21 days that occurred in 1980 and 1988. 

High temperatures today could reach 90°.  Starting tomorrow high temperatures are expected to be in the low 90s.  If the current heat wave doesn’t end today, then the existing record of 21 consecutive 90° days will be broken by a considerable margin.  That’s because after today, highs are expected to be in the 90s for the next seven to 10 days in the DC Metro Area.

The primary difference between DC’s current heat wave and other memorable ones is the lack of extreme heat.  The hottest temperature during the current heat wave was DC’s high of 97° on July 3.  It’s rare for Washingtonians to experience a heat wave lasting 10 days or more without any triple-digit heat.  Before last summer’s 12-day heat wave (July 11 – 22) when DC’s hottest temperature was 99° on July 21, the last time DC residents experienced a heat wave of at least 10 days without any triple-digit heat was a 14-day stretch from July 23 – August 5, 1995.

The current heat wave isn’t that extreme compared to earlier ones.  DC’s average high during the current heat wave is 92.1°.  That’s compared to 94.2° during the 1995 heat wave and 93.8° last summer.  The 18-day heat wave from July 22 – August 8, 1999, had an average high temperature of 94.4° with a peak temperature of 101° on July 31.

Longtime Washingtonians may recall the very hot summers from 2010 through 2012.  The longest heat wave during that three-year period was a 16-day stretch from July 18 – August 2, 2011.  Highs in the Nation’s Capital reached 100° four times, with the average high during that heat wave being 97.4°.  The hottest period during that three-year stretch was an 11-day heat wave from June 28 – July 8, 2012.  DC’s average high during that period was 99.6° as temperatures reached 100° or higher five times.

These examples illustrate that while the current heat wave could set a new record for DC’s longest, it has hardly been extreme.  Having a high temperature in the low 90s is barely warmer than average in July, but does meet the definition of a “heat wave.”  The last time it was 100°/+ in the Nation’s Capital was August 15, 2016.