Hot Hot Heat Why summer in D.C. is only getting warmer.

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.

Our Readers Say

Lois - thanks for this great article. Just wanted to add that there are steps we can take to help reduce the urban heat island effect. For example, installing a white roof when your current roof fails is a simple way to help reduce urban heat and it costs about the same as a dark roof. The reductions in cooling energy don't hurt either.
After reading your article, I went out and purchased two thermometers, took them up to my roof, and suspended them from tripods about a foot above the surface.

I put one in the middle of the half of my roof that is covered by a living green garden, and the other in the middle of the half that is covered with a silvery tar-like substance.

Then I took readings, and kept track of the results.

In general, I found the green side was always cooler, ranging from the same (at night) to 23 degrees cooler just after a watering.

Using back-of-the-napkin calculations, it appears the chemical roof was on average hotter by six degrees than the green roof when the sun was directly overhead, and between zero and six degrees hotter relative to how high or low the sun was in the sky throughout the day--assuming a sunny day.

(Here are my actual readings:

From sunup to sundown, then, the extra heat being generated on the non-green roof appears to be 42 degrees per square foot (one degree between 6am and 7am, two degrees between 7am and 8am...six degrees during each of the hours on either side of high "noon", and so on back to sundown).

Since the green roof and the chemical roof both cover about 725 square feet, my crude calculations seem to suggest that the non-green half of my roof throws an astonishing 30,450 *extra* degrees of heat into the atmosphere every sunny day--something like 6,000 BTUs of heat since it takes about .02 BTU to raise the temperature of one cubic foot of air one degree.

Assuming I'm not missing something that makes my conclusions wildly off, that's a whole lot of extra heat from just my building.

A quick look at Google maps satellite view shows my building accounting for something like a half percent of the hard surface area in my block. To make the leap: you could say the presence of non-green surfaces in my block produces *extra* heat roughly equivalent to running ten gallons of jet fuel through a jet engine in my block every day.

And the block next to mine. And the next block after that.

I can't help but think it would make a huge difference in the city (not to mention the planet) if there were a real effort to get gardens on our roofs.

I made my garden almost entirely out of discarded Christmas trees. Thus it was extremely cheap. It's secure, having withstood a hurricane last year, and a nasty windstorm earlier this year. It's lightweight and drains extremely well, alleviating two major reservations landlords often have about a green roof. (I hereby nominate my landlord, Pat Welch, for the Mayor's Environmentalist of the Year award for letting me try it out in the first place.) And, I have flowers and vegetables on my roof!

At some point, I'm going to try to get a video up about how I built it, so others can use my technique if they want. But, in the meantime, I'm happy to show it to anyone who asks.

Hopefully, the city government will consider tax incentives or something in the near future to encourage others to help cool the city by making their roofs green. And, who knows? If we all paid more attention to how green our city is, it might seem less black and white.

Craig Nelsen
Blagden Alley
Washington, DC

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