City Hot and Cool Spots Remain a Mystery

Friday, July 23, 2010

It’s no secret that cities are often hotter than their surrounding regions. For one thing, they’re crowded with warm bodies. And they contain lots of machinery to power our lights and keep our cars and transit systems moving.

“Just about everything we do when we build cities unfortunately traps heat, reduces wind, dries them up, essentially reduces soil moisture,” says Stuart Gaffin, an atmospheric physicist at Columbia University’s Earth Institute. Scientists call this the urban heat island effect. But that effect isn’t uniform. Gaffin has studied different locations around the city to look for tiny variations. “And one of the surprising things we did find is that some neighborhoods are actually cooler than the rural, distant comparison areas at certain times of day,” Gaffin says.

Gaffin and his team of researchers analyzed temperature readings from five rooftop weather stations in Brooklyn, the Bronx and Manhattan over the course of a year. They compared them to Central Park and to eight stations in a wide circle around the city. They discovered a mystery. The location in East Flatbush, bordering Crown Heights, was consistently cooler during the late afternoons. But nearby Bensonhurst, near Ocean Parkway, was the hottest, often by a couple of degrees Celsius--or as much as five degrees Fahrenheit.

“I tried really hard to see obvious things that might help explain it,” Gaffin says. “I looked at things like population density, the amount of vegetation, trees and nothing really stood out, obviously.”

The hotter neighborhood also consumed more power per capita. His study was published in an international climate journal and acknowledged the unanswered questions. Gaffin suspects wind speeds might have played a role but that needs further study.

Using a completely unscientific method, WNYC compared the two neighborhoods this week. Armed with thermometers, intern Daniel Tucker and I stood outside the weather stations, which were located on top of Catholic schools. I was in Bensonhurst.

At 3:15 p.m. I got a reading of 92 degrees. Dan's thermometer in East Flatbush, shaded by his own shadow, read about 88-89 degrees. 

So, it was true. Or at least at that moment in what could at best be called a demonstration. We weren’t even using the same type of thermometer.

Maybe it was the strength of the sun that day. But in both locations, people told us they thought they were living in the hottest neighborhoods. One local resident, Marilyn, who declined to give her last name, was surprised to hear East Flatbush is often as cool as the suburbs.

"The ocean, it’s not close, there’s not so much trees like other areas," Marilyn says.

In Bensonhurst, Angela Quaranta offered her own theories for why her neighborhood is so hot.

"I think it’s pretty hot because we don’t have tall buildings and we’re not close by the water, unless you go to Coney Island then it’s cooler over there," Quaranta says.

At the Earth Institute, Stuart Gaffin has since moved on to using weather stations on green rooftops. He thinks these are more accurate because the grass buffers the thermometers against any heat from the buildings. He’s found big differences between Morningside Heights and Riverdale, which is cooler and shadier. Gaffin hopes his work can help city planners figure out how to address the hot spots.

"I’m just guessing they’re going to be the weak links in our electricity grid during heat waves. The heavy loads will probably be in those areas. I would prioritize tree planting programs in those locations. I would prioritize cool roofing and green roofing programs in those locations," Gaffin says.

The city has a goal of planting a million trees by 2017. It’s given priority to areas with low canopy covers, including Hunts Point and East Harlem. Those planners might now want to take a look at Bensonhurst.

Stephen Nessen/WNYC
A study by researchers at the Earth Institute at Columbia University found that the rooftop weather station in Bensonhurst consistently had higher temperatures than the weather stations at four other
Beth Fertig/WNYC
Stuck with an infrared thermometer, WNYC’s Beth Fertig got creative and pointed it at a piece of paper shielded by a hat when she went to Bensonhurst. She took the temperature while holding the paper

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