summer in the city

Dale Quattrochi likes to tell people that parking lots wreak havoc on the weather. He points to the man-made stuff all around–asphalt, concrete, tar, metal–and explains that because of parking lots, streets, and sidewalks, because of the roofs of cars, buildings, and houses, because of all this dark stuff coating urban surfaces, cities are much hotter than they should be. These materials absorb the sun’s energy and concentrate its heat; above every city in the world rises an invisible plume of heat, an “urban heat island” several degrees hotter than the surrounding area. These higher temperatures devastate air quality. They provoke thunderstorms in clouds that would otherwise be placid. They make life less bearable and cost millions of dollars in electricity every year.

People have known about the heat island effect since an amateur climatologist named Luke Howard walked around London with a thermometer in 1818. Since then, scientists have found that cities are heated by a lot of very small things spread all over town. Of course, no one has been able to determine how much heat every object adds; such an undertaking would be impossible. But, according to Quattrochi, only a thorough analysis of an entire city can provide the kind of information that urban planners, architects, and others can use to cool cities off.

Read the article on the Metropolis site or Quattrochi is in a rare position to create just such a detailed inventory. As a geologist at NASA’s Global Hydrology and Climate Center in Huntsville, Alabama, he works for the institution most able to create comprehensive images of the energy given off by the surface of a planet. “We’re using the technology that NASA has developed to look at outer space, and focusing it back here on Earth,” he says.

The technology–namely the Advanced Thermal and Land Applications Sensor, or Atlas–sits on the belly of an airplane and takes high-resolution infrared photographs of the energy given off at the planet’s surface. (It’s the same technology NASA has used to study the surfaces of Mars and Jupiter.)

Analyzed, Atlas images show exactly how much heat is released or reflected by every house, building, block, stretch of road, or other object more than 10 meters square. The data is plugged into a computer operated by Haider Taha, a climatologist at the Lawrence Berkeley National Laboratory (LBNL), who can then play endless games of “What if?” What would happen to a city’s temperature if its roofs were all painted white? If the number of trees in town doubled? Tripled?

Quattrochi, Taha, and six other scientists from NASA, LBNL, and several universities around the country make up part of the Urban Heat Island Pilot Project (UHIPP), sponsored by the Environmental Protection Agency. This year, they’ve flown Atlas over Baton Rouge, Salt Lake City, and Sacramento, three cities that have had problems with air quality and need help in lowering their temperatures. (Last year, in a related project, Quattrochi ran Atlas over Atlanta.) In 1999, they will use Taha’s computer modeling to explore the heat sources of Chicago, Houston, Los Angeles, Phoenix, Tucson, and Nashville.

If UHIPP had begun just a few years ago, the data it collects would have been used by only a handful of scientists in the obscure field of urban climatology, says Jeff Luvall, a NASA staff scientist and the principal investigator on the project. “Until recently, applications was a dirty word at NASA,” he explains. “You just did your work for the science and didn’t worry about how it filtered out to the public. You just did your journals and that was it.”

Not anymore. Thanks to a recent major shift in policy, NASA is no longer gazing only at the heavens but is seeking ways to use its resources to improve life here on Earth. To that end, Quattrochi and Luvall hired Maury Estes, an urban planner, to make sure that the data collected by UHIPP is translated into terms useful to planners, architects, legislators, and homeowners. Estes wants to make sure that everyone who can use the data has access to it; he has already contacted dozens of not-for-profit advocacy groups, city and state planners, utilities, and others in the three cities UHIPP is studying this year. By the end of 1998, he hopes to package the data these groups and individuals want on a CD-ROM, free to anyone who asks, and on a Web site (

To use the data, though, laypeople first have to learn just what causes urban heat islands. Light-colored surfaces and plant life don’t raise a city’s temperature. When the sun’s energy hits anything painted white it’s reflected back into the atmosphere or out to space. And when the sun’s light hits water or vegetation, much of its energy is used to convert water into vapor, which then cools the air.

When light hits dark materials like asphalt or tar, though, most of its energy is converted to heat. Some heat is released right away–which is why asphalt parking lots get so hot during the day–while some is stored for a few hours, raising temperatures at night. Overall, cities usually run 8 to 10 degrees Celsius hotter than surrounding rural areas; smaller areas are even hotter. “I had a handheld infrared thermometer in a parking lot in August in Atlanta,” Luvall recalls. “We found a black cloth roof of a Corvette which was 90 degrees Celsius. Right next to it was a white car that was 30 degrees.” (194 and 86 degrees Fahrenheit, respectively.)

Urban heat islands cause more than personal discomfort. Higher temperatures create a greater demand for energy, forcing utilities to build more power plants, which create more pollution. And the heat causes reactions in the air that convert existing chemicals into smog.

The UHIPP data, packaged for the nonscientist, will reveal what parts of a city add the most heat. And Taha’s work at LBNL will show what can be done to lessen their effects. But, Quattrochi says, the things that will make cities cooler are simple: plant more trees, paint roofs and parking lots navigation_elements/white. The UHIPP team doesn’t know yet how many trees should be planted or how many surfaces painted, but they hope more work with Atlas and the LBNL computer will bring them closer to the answers.

Quattrochi recognizes that painting all the roofs of a city would be absurd, and that some cities don’t have much room for more trees. “Central to this project is the idea that cities will continue to develop,” he says. “We hope that this data will be used to plan new development in the future. We want to learn how to make cities more comfortable, more healthy, more highly desirable places to live.”