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°)