THE LEAFLET

Remember when we called for volunteers to help out with a NOAA driven study on temperature difference throughout the city? And then it rained for (what felt like) the entire month of July? Well, an upside of that late breaking, ferocious heat wave in late August? We finally got to study the urban heat island effect on our city! In the image above, Jeremy Hoffman attaches a special thermometer to the passenger door of his car to monitor temperatures as he drives around the District.

Jacob Fenston has the full scoop, from the DCist:

You might think that one person who would appreciate weather above 95 degrees — with humidity making it feel like 105 — would be a guy selling ice cream.

But alas.

“If too much hot, no business,” says Habtom Mengesha, whose ice cream truck was parked by the National Mall on a scorching afternoon this week. On days like this, he says, no one buys ice cream, because everyone’s inside. Business is so slow, he got out of the truck and walked over to the shade of an old elm tree.

It’s noticeably cooler, away from the asphalt road, and the concrete and gravel paths radiating heat, and noticeably empty of tourists. “Yeah, I feel good here,” says Mengesha, standing under the tree.

This temperature difference is a small example of a phenomenon scientists call the urban heat island effect. This week a group of scientists and volunteers fanned out across the city to measure this heat island effect, and map temperatures across Washington — there could be as much as a 10, 15 or 20 degree disparity between the most heat-baked zones and the coolest, shadiest areas. That could have big implications for public health, especially in a warming climate, where hot weeks like this one will become much, much more frequent.

“The urban heat island effect is basically all of those things that define human civilization,” says Jeremy Hoffman, climate and earth scientist with the Science Museum of Virginia, one of the organizations behind the heat mapping project.

“Buildings made out of dense dark surfaces like brick and asphalt — those things absorb more of the sun’s energy during the day and then re-emit that energy back into the atmosphere as heat for a lot longer than the sun is out,” he says.

These manmade building materials retain heat, amplifying temperatures during the day, and keeping temperatures high into the night.

To map D.C.’s heat islands, the research team recruited volunteers to drive around the city three times on one of the hottest summer days. There are nine cars, following different zig-zag routes through D.C., each equipped with a GPS unit and a fancy thermometer that sticks out the car window and logs temperature every second.

Each driver travels a one-hour route — first at the coolest part of the day, starting at 6 am, then at the hottest part of the day, 3 pm, then again at 7 pm.

The data to create the map is still being processed.

Scientists have known about the urban heat island effect since the 1800s, and D.C.’s heat islands can be seen on satellite images from NASA. The D.C. Policy Center studied one such image, from August 2015, when the recorded temperature was 93 degrees. The satellite showed that areas with little tree cover, like Ivy City and Trinidad, registered temperatures of 102 degrees, while in Rock Creek Park, it was just 76. Hoffman says this effort will create, for the first time, a much higher resolution map of where D.C. is the hottest. The satellite images, he says, are like looking at the world with your glasses off.

Why do we need to see a detailed heat map of the city? Here’s one example. Last summer, Hoffman and others mapped the heat in Richmond, Virginia (where they found a 16-degree difference between the hottest and coolest areas), then overlaid that map with data about heat-related emergency calls from the Virginia Department of Health and the Richmond Ambulance Authority.

“Those areas that the heat was the most concentrated in are also those areas where the local ambulance authorities goes to respond to people calling for an ambulance for a heat-related illness,” says Hoffman.

They also found a strong relationship between heat and socioeconomic factors.

“People with lower income, living in poverty, have lower educational attainment, may be renting their home, person of color, those are the people that are experiencing the warmer temperatures more often.”

The project is also backed by researchers at Portland State University, who also created a heat map of Portland, Ore., and found people and businesses located in the hottest areas were paying hundreds of dollars more in energy bills.

These hotspots will only get worse with climate change, says David Herring, who is with the climate program office of the National Oceanic and Atmospheric Administration. NOAA is funding the heat mapping project.

Herring says in the 20th century, D.C. averaged about 5 days a year over 95 degrees.

“By the year 2030, we could have 20 days out of the year in which temperatures exceed 95 degrees. By the year 2050, that number is projected to be around 40 days out of the year. And then by the end of the century about 80 days of the year.”

It’s not just that heat is uncomfortable. It’s dangerous. Heat is the number one weather-related killer, causing more deaths each year than floods, tornados or hurricanes.

A detailed heat map of the District may help city leaders keep it a bit cooler.

Kate Johnson, with the District Department of Energy and Environment, says there are two big ways to think about cooling hotspots in the city (besides, of course, trying to slow down global warming). You can make the city more reflective, for example, making rooftops with silver or white, rather than black (this is a requirement for new buildings in the District).

“That’s great because it helps keep your building, your home cooler, and it helps keep the surrounding area cooler too,” says Johnson.

The other option is to make the city greener, putting in green rooftops, replacing pavement with green space, or planting trees. Johnson says she’s looking forward to seeing the heat map researchers are working on — it will show where these things are most needed.

One positive note: as D.C. grows, new development may actually help mitigate the heat island effect, says Johnson.

“In the past, what you saw was that as cities grew and densified, they paved over a lot of things and that’s what actually created urban heat islands in the first place,” says Johnson.

But now as the city grows, many new buildings are replacing heat-retaining things like surface parking lots, and because of the District’s environmental laws and building codes, new buildings have features making them cooler than old ones.

Scientists Are Mapping D.C.’s (Literal) Hottest Neighborhoods by Jacob Fenston for WAMU.