Credit: Photograph by Darrow Montgomery

The afternoon of June 29, the tar on Washington, D.C.’s streets was melting. Citizens and tourists alike collapsed inside in front of air conditioners. Corner stores sold out of ice. When thunderheads began to gather just before dusk, people breathed a sigh of relief. A quiet gloom fell over the city’s neighborhoods, shading small square lawns that had been browning in the heat. But when the line of storms arrived, as the lightning cracked and the deluge began, the mercury crept past 104—the hottest day in June on record. The whip of the derecho’s wind blew in undulating patterns, the kind of gusts you can see in the rain. At Reagan National Airport, the weather station reported wind speeds of 75 miles an hour. The next morning, after 22 people died, 4.3 million houses in the region lost power, and Mayor Vince Gray declared a state of emergency, the heat still had not broken.

This summer, “cooling” buses have set up in front of elementary schools and the National Oceanic and Atmospheric Administration’s weather advisory has regularly declared warnings for anyone “spending time outdoors.” June and July’s heat waves set a string of new highs quantifying the District’s misery, including records for the most consecutive days over 100 degrees (four), the all-time hottest daily average temperature (July 7, at 94 degrees), the second hottest temperature of all time (July 7, at 105 degrees), and the longest stretch of highs at or above 95 degrees (11 days).

We’ve all heard about climate change, of course. But there’s another reason D.C.’s heat has become so oppressive, one that regularly elevates temperatures dramatically.

In 1952, a man by the name of Tony Chandler attached a thermometer to the front of his Land Rover and drove around London, pulling over to the side of the road periodically to peer over the hood and scribble down the temperature. The survey he published after two years of driving in circles was the first spatially-focused climatological study of its kind. London, Chandler said, was the victim of something he termed the “heat island effect.”

According to Marc Imhoff, Terra project scientist at NASA, Chandler’s discovery explains why D.C.’s downtown temperatures now regularly climb up to 15 degrees higher than the surrounding areas. The effect is caused by man-made materials such as concrete and asphalt, which absorb sunlight, keeping heat in; it’s the city’s equivalent of wearing a black shirt to a picnic. These non-porous materials also minimize evapotranspiration, a process by which plants sweat water into the air, nature’s version of air conditioning. Add to that tall buildings that block wind, and factors like traffic and industrial cooling systems that produce waste heat, and the differential temperatures can get extreme. Even temperatures across neighborhoods can vary widely because of the heat island effect, depending on the density of trees. Paul R. Baumann, a geographer at the State University of New York at Oneonta, found that across three sections of Bethesda, the average temperatures varied by as much as 10 degrees, with the coolest neighborhoods being the most densely vegetated. (Urban sprawl opponents will be sorry to hear that rambling, less densely developed areas have smaller heat islands.)

Unfortunately for D.C. residents, the effect not only keeps urban areas hotter, it also increases the time it takes air to cool again—sometimes hours, sometimes into the next day. While previous studies have found the effect most noticeable at night, a new report by Imhoff used satellite measurements to determine that the largest differences in urban areas like D.C., where city replaced forest, were actually midday.

D.C. is not the only city with a heat island—most urban landscapes have one—but how large and how drastic the effect is depends on the location of the city and its urban landscape. Because of the city’s geography, D.C.’s heat island effect is particularly dramatic. A new report by NASA analyzing 42 cities ranked D.C. among the worst, with an average citywide increase of 13 to 16 degrees.

With climate, every fraction of a degree counts. Bruce Hicks, former director of the Air Resources Laboratory at NOAA, explains that even a little extra heat in the system will drive “meteorology with ever increasing vigor. I liken it to a violin string. Pluck it a little, and it will vibrate only a little,” he says. “Put more energy into it, and it will oscillate with greater amplitude.”

The heat island effect only increases temperatures locally, but along with climate change this additional energy is having a profound effect on the District’s weather. Yes, during the Snowpocalypse two years ago, climate change skeptic Sen. James Inhofe (R-Okla.) went so far as to build an igloo on the National Mall labeled “Al Gore’s new home.” But Inhofe may not be laughing as he sweats through this summer. Scientists have since explained that the warming atmosphere is causing storms—during both summer and winter—to become stronger and more frequent. “Hots are getting hotter and the colds are getting colder,” Hicks says. “There is more heat energy being retained within the atmosphere. Every aspect of weather is getting more energetic.”

So this summer’s heat waves may not be an outlier, but a prediction of our future. In a warming era where summer temperatures regularly top 100 degrees, heat islands are particularly dangerous because, as Benedicte Dousset told NASA, it’s “the lack of cooling at nighttime, rather than high daytime temperatures, that poses a health risk.” A new study by the Natural Resources Defense Council puts the repercussions of this trend in more immediate perspective: More than 150,000 additional Americans could die by the end of this century due to excessive heat. And it’s not just people who are affected by the rising temperatures. Botanist Stanwyn Shetler has found that 89 out of 100 species of plants in D.C. are flowering earlier, including the famous cherry blossom trees, which bloomed a full week earlier in 2000 than they did in 1970.

During another heat wave in August 1787, Elbridge Gerry wrote to his wife from the Constitutional Convention in Philadelphia. “Mr. Martin I saw…he rode from Trenton in the forenoon and had nearly fainted when he dismounted, on account of the heat.” There’s a reason Congress takes an August recess—but this year D.C. residents are facing higher, more sustained temperatures than the founders ever had to deal with.