Does triple-digit heat guarantee a hot month?

 

A summer's day in Washington, D.C.

The nation’s capital has already had three days of triple-digit heat so far this month. Such heat is a relatively rare in the DMV (DC, Maryland and Virginia). Although July is the hottest month of the year in the nation’s capital – based on average temperatures – triple-digit heat doesn’t guarantee that a month will finish with above average temperatures.

Last July, for example, finished as DC’s tenth warmest on record. NOAA weather averages in the nation’s capital date back to the 1870s so finishing in the Top 10 is an achievement. However, DC’s hottest temperature all of last July was 98°. What made July 2025 so much warmer than average was how warm its overnight low temperatures remained. DC’s average daily low temperatures last July finished 2.1° above average.

Warmer overnight low temperatures can help offset high temperatures that aren’t particularly warm. That’s happened more recently this century as warmer overnight low temperatures have become commonplace. The combination of long stretches of unusually oppressive weather and the urban heat island effect have also helped this month’s low temperatures remain warmer than they otherwise would be.

The urban heat island effect has become more pronounced in the nation’s capital in recent decades as automobile traffic has increased. There has also been a commensurate reduction in green space as asphalt and buildings have become more concentrated.

Last July was the second July this decade to rank among DC’s Top 10 warmest without any triple-digit heat. Decades past, however, have had months with triple-digit heat that not only didn’t rank among the warmest months in the nation’s capital, but in some cases finished as cooler than average months. Such was the case during the summer of 1997. A total of five days of triple-digit heat occurred that summer with some in June, July and August that summer, but all three months were cooler than average. That helps underscore that a few very hot days don’t guarantee a hot month.

DC's warmest July's based on monthly average (Source: NOAA)

1. 84.5° (2011)  (Average high: 93.6°, Average low: 75.4°)
2. 84.0° (2012)  (Average high: 93.1°, Average low: 74.9°)
2. 83.9° (2020)  (Average high: 92.7°, Average low: 75°)
4. 83.7° (2024)  (Average high: 92.4°, Average low: 75°)
5. 83.1° (2010, 1993)  (Average high: 92.3°, 92.5°, Average low: 73.9°, 73.7°)
6. 83.0° (1999)  (Average high: 92.8°, Average low: 73.1°)
7. 82.7° (2016)  (Average high: 91.2°, Average low: 74.3°)
8. 82.6° (1987)  (Average high: 92.2°, Average low: 72.9°)
9. 82.4° (2025)  (Average high: 90.3°, Average low: 74.5°)
10. 82.3° (1980)  (Average high: 91.0°, Average low: 73.6°)

Average: 81° (Average high: 89.6°, Average low: 72.4°)

A closer look at some of DC’s impressive July rainfalls

 

A rainy July 2019 day in Rockville, Maryland (Photo Credit: Susan Granzow)

NOAA’s (National Oceanic and Atmospheric Administration) current 30-year average temperature and precipitation totals for the United States reflect the period of 1991-2020. Since this period was appreciably warmer and wetter in the nation’s capital than the previous 30-year period of 1981-2010, there were significant changes. 

July saw the most significant changes in the updated data set. Previously, July was DC’s third wettest month of the year with an average of 3.73” of rain. However, July is now the wettest month of the year in the nation’s capital with a new monthly rainfall average of 4.33”. 

Here’s a look at recent rainy July’s in the DMV (DC, Maryland and Virginia).

2024: Although this July is remembered for its extreme heat, it also had very wet weather. The four-day period of July 22 – July 25 had two days with more than an inch of rain in the nation’s capital (July 22: 1.57”; July 25: 1.28”). While not record-setting, these impressive daily rainfall totals helped the month finish with above average rainfall.

2022: This July continued the recent trend in the nation’s capital for exceptionally wet days. A total of 4.05” fell on July 9, which was DC’s fifth wettest July day. To add to that, a daily rainfall record was set merely a week later on July 16 with another 1.38” when thunderstorms passed through the DMV.

2020: The monthly rainfall total of 6.51” was above average. What made this July significant was that 60% of the monthly rainfall occurred on just two days. A daily rainfall record of 2.04” occurred on July 7, while another 1.89” fell on July 23.  

2019: This July got off to wet start with measurable rainfall on five of its first eight days. The signature rainfall event was on July 8 when a daily rainfall record of 3.44” occurred at National Airport. That accounted for more than half of DC’s monthly total of 6.49”. Most of that rain fell in less than two hours, snarling the morning commute across the DMV.

2018:  An impressive total of 4” of rain fell at National Airport on July 21 that set a new daily rainfall record. Moreover, 8.52” of rain fell during the eight-day period from July 17 – July 24. It finished as the fourth wettest July in the nation’s capital with 9.73”.

2017:  This July finished with 9.15” of rain, making it DC’s wettest since 1969. During an eight-day period from July 22 – July 29, a total of 6.15” of rain fell. The wettest day of the month was on July 28 when a daily rainfall record 3.31” was recorded at National Airport.