Sunset, Aug. 24. ( George Jiang via Flickr )

It’s hot and getting hotter. Our analysis of summer temperatures at all three D.C.-area airports (Reagan National, Dulles and BWI) reveals an unmistakable increase in average and extreme summer temperatures.

Heat waves are becoming more intense, and it’s staying hot for longer periods of time. Remarkably, the hottest four summers on record dating back to 1872 have occurred since 2010 (if you include 2016).

Average summer temperatures

In the past 50 years, from 1966 to 2015, the series of temperatures at each of the three airports show a rise.

(Note: we chose to analyze the past 50 years because it represents a period of time during which measurements have been continuously maintained at all three airports; Dulles did not exist before 1963, and Reagan National and BWI’s temperatures measurements were taken at different locations before the 1940s.)

Dulles has warmed the most of the three locations, by roughly 2.5 degrees. Although D.C. and Baltimore have warmed, as well, their changes of 1.4 and 1.0 degrees respectively are more modest.

Dulles — easily the coolest of the locations in the 1960s — has become nearly as warm as Baltimore in recent years, perhaps reflecting urbanization creeping into Loudoun County.

Warm overnight lows are increasing

During the region’s heat waves, data show overnight temperatures are not cooling down as much as they used to. We see this in the number of days when the temperatures do not drop below 75 degrees.

Mornings with lows of 75 degrees or higher are on the rise at all three airports.

In D.C., we’ve seen an increase from an average of about 11 such days per year in 1966 to around 19 per year now.
Such warm nights are less common at the more rural Dulles and BWI locations but are also increasing there. At Dulles, 75-degree lows seldom happened a few decades ago but are typically an annual occurrence now (an average of two per year). In Baltimore, they have doubled in the past 50 years (from two to four).

Hot days are increasing

The region is seeing scorching afternoon temperatures of at least 95 degrees more frequently.

Both D.C. and Baltimore began the past 50-year period with expectations of around seven 95-degree days per year. Since then, D.C.’s typical has doubled to 14, while Baltimore has seen a slower rise to around 10 per year. At Dulles, the rise has been from roughly five to nine such days over the past five decades.

(It is worth noting that the trend in 95-degree days is sensitive to start date; changing the start date can lead to lesser or greater trends.)

Extremely hot days and warm nights are increasing in D.C., sharply since 2010

Perhaps the most astonishing temperature trend locally has been the increase in low temperatures at or above 80 degrees in D.C. — especially if we extend the time series back to 1872 (note that before 1945, temperature measurements for D.C. were recorded downtown rather than at Reagan National).

Since 1872, including this year (when we’ve had a record-tying seven), there have been 59 80-degree (or higher) lows on record. Almost half of them (47 percent) have occurred since 2010 (28).

High temperatures of at least 100 degrees have also seen a big rise since 2010. Of the 121 days since 1872 at or above 100 degrees, 21 have occurred since 2010 — or roughly 17 percent.

The number of 100-degree highs and 80-degree lows since 2010 are well above what we should expect in any decade per historical precedent, but the 80-degree figure is particularly notable.

We have logged an astonishing list of warm weather records and milestones since 2010 in Washington, which is not confined to summer:

(Capital Weather Gang)

Sustained summer warming extends back to the late 1800s in D.C.

Summer temperature rises locally have been pronounced in D.C. across the entire record. All categories of summer temperature averages are up several degrees since the 1870s.

The June-August temperature average has risen about 4.5 degrees since the early 1870s. High temperatures have shown the smallest increase, although the more than 3-degree rise there is still rather substantial.

Lows, which may be affected the most by the urban heat island effect, have risen the quickest. Over this time frame, they have warmed 5 degrees.

It is true that the D.C. recording station moved from downtown to a few miles south of downtown in the 1940s, although temperature trends appeared to be similar at the two stations while they were both up and running from 1942-1945 (the D.C. location was a hair cooler than National). Similarly, since we have installed our weather station in downtown Washington at The Washington Post, it has tracked the measurements at Reagan National fairly closely.

We acknowledge that temperature records maintained at the airports are not perfect. Sometimes, the temperature sensors have been documented to report erroneous values, for example, and have needed to be replaced. Questions have also been raised about land-use change around the observing locations, especially at Reagan National, and the impact it might have on temperatures.

Even so, the trends we see in average and extreme temperatures at all three observing locations leave little doubt that the climate is warming. Increased urbanization in the region certainly plays a role, especially for low temperatures. Climate warming from increasing greenhouse gases is very likely also contributing to the rise in temperatures. Untangling the relative contributions to warming in the region is beyond the scope of this analysis.

All data via xmACIS2.