The hottest time of the year?

 

Dangerous July heat in the DMV

July is the hottest month of the year in the nation’s capital based on average temperatures kept by NOAA. The hottest days are from July 11 - 27, when average daily high/low temperatures are 90°/73°. NOAA’s climate averages are based on a 30-year dataset that’s updated every decade. NOAA updated its averages in 2021 to reflect the period from 1991-2020, which replaced the previous data set from 1981-2010.

Since the 2010s were a much warmer decade than the 1980s, DC’s average daily high temperature increased from 89° to 90° for most of the month. That should not surprise longtime Washingtonians as seven of DC’s 10 hottest July’s have occurred since 2010. Consequently, DC’s average July temperature (combining daily high and low temperatures) for the 30-year period from 1981-2010 was 79.8°, but rose to 81.0° when updated to reflect the more recent time period.

More triple-digit heat has occurred in July than any other month in the nation’s capital. All but three July days have seen triple-digit heat since official weather records began in 1871.

This month got off to a torrid stretch in the nation’s capital with a rare stretch of triple-digit heat on three consecutive days (July 2 – July 4). That was followed by two more days in the upper 90s on July 15 and July 16. Although July 2026 got off to an exceptionally hot start, it will not rank among DC’s Top 10 warmest due to the near to below average temperatures that have been prevalent during the second half of the month.

Weather observations in the nation’s capital are made at National Airport. A southerly wind at National Airport blows off the cooler waters of the Potomac River. That tends to keep the air temperature a few degrees cooler during summer than downtown areas away from the river. That doesn’t mean, however, it’s any less humid or uncomfortable.

The major takeaway is that while triple-digit heat is relatively rare in the nation’s capital, the most likely time for it to occur is in mid-to-late July given historical weather data. But triple-digit heat isn’t necessary for there to be dangerously hot conditions in the DMV (DC, Maryland and Virginia). Drinking plenty of water, doing outdoor activities early in the day when its cooler, and applying suntan lotion regularly are all good summer safety tips.

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°)