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.

Is DC’s rainy summer unusual?

 

I-270 during record July rainfall in 2019 (Source: Susan Granzow)

July is the wettest month of the year in the nation’s capital according to NOAA, with a monthly rainfall average of 4.33” based on the 30-year period of 1991-2020. In recent years, July has been a particularly wet month. 

Since 2017, the nation’s capital has had 14 July days with at least an inch of rain, including four days with at least 3” of rain. July isn’t usually an active month in the tropical Atlantic so most of this rainfall occurred absent any direct influence from a tropical system.

The irony about this July is the rainy weather pattern that began across the DMV (D.C., Maryland and Virginia) in May has continued for more than two months. That followed an extended period of below average rainfall in the nation’s capital that led to widespread moderate to severe drought conditions.

May 2025 finished as D.C.’s ninth wettest with 15 days of measurable rainfall, including two days of over an inch of rain. June was also wetter than average with another two days more than an inch of rain was measured at National Airport. The wettest day so far this month was July 9 with 1.16”. The good news is the drought conditions no longer exist, but the frequent and sometimes heavy rainfall has led to flooding in parts of the DMV.

None of D.C.’s rainfall since May has produced any daily rainfall records. Despite the rainfall surplus of nearly 5” since May 1, the nation’s capital hasn’t had a day with at least 2” of rain in more than 18 months. That’s unlike recent July’s that have featured heavy, record-setting rainfall. Nevertheless, localized heavy rain can lead to dangerous flooding events such as what occurred in parts of Silver Spring and Takoma Park, Maryland on July 19.

D.C.’s 10 wettest July days (Source: NOAA)

1. 4.69” (July 9, 1970)
2. 4.35” (July 22, 1969)
3. 4.25” (July 30, 1878)
4. 4.12” (July 30, 1876)
5. 4.05” (July 9, 2022)
6. 4.00” (July 21, 2018)
7. 3.68” (July 5, 1908)
8. 3.44” (July 8, 2019)
9. 3.37” (July 13, 1922)
10. 3.31” (July 28, 2017)

D.C.’s 14 July days with 1”/+ of rain since 2017 (Source: NOAA)

1. 3.31” (July 28, 2017)
2. 4.00” (July 21, 2018)
3. 2.79” (July 17, 2018)
4. 3.44” (July 8, 2019)
5. 2.04” (July 7, 2020)
6. 1.89” (July 23, 2020)
7. 1.27” (July 1, 2021)
8. 1.15” (July 9, 2021)
9. 4.05” (July 9, 2022) à Second wettest July 9
10. 1.38” (July 16, 2022)
11. 1.33” (July 14, 2023)
12. 1.57” (July 22, 2024)
13. 1.28” (July 25, 2024)
14. 1.16” (July 9, 2025)

DC’s hottest time of the year?

 

The White House, Washington, D.C.

Washingtonians often experience some of the hottest weather of the summer during mid-to-late July. That’s when average daily high temperatures are at the highest of the year at 90°. The weather pattern is often such that hazy, hot and humid conditions dominate local weather headlines.

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. There are three days in mid-July that 100-degree heat has occurred five times in the nation’s capital: July 16, July 17, and July 21. 

There’s been no triple-digit heat so far this summer. That’s in contrast to last summer that featured six days of triple-digit heat and finished as DC’s third hottest summer. Last summer was also DC’s first summer since 2011 that 100-degree heat occurred in three consecutive months (June, July and August). 

As hot as last summer was, it also helped illustrate how rare 100-degree days are in the nation’s capital. Before last summer no triple-digit heat had occurred in Washington, D.C. since August 15, 2016. The nation’s capital averages only one to two days of triple-digit heat every four to five years.

High temperatures in the mid to upper 90s are more common than triple-digit air temperatures are in the nation’s capital. Washingtonians saw highs of 99° during the summers of 2019, 2020 and 2022 during the nearly eight-year streak without triple-digit heat. It has already been as hot as 99° so far this summer as well.

Weather observations for 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. Some Washingtonians lament how humid it is in the nation’s capital during the summer, as opposed to the southwestern United States where it’s hotter, but a “dry heat.”

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 dangerously hot conditions in the DMV (D.C., Maryland and Virginia) given how oppressive the summer months often are. Drinking extra water, doing outdoor activities early in the day when its cooler, and applying suntan lotion are all good summer safety tips.