It's long been known that urban areas generate and hold heat. But now two University of Minnesota researchers are trying to uncover the devilish details about the phenomenon, to help develop ways to reduce energy use and protect human and animal health.

Atmospheric science professors Peter Snyder and Tracy Twine are looking to place 200 temperature sensors around the Twin Cities -- in grassy back yards and bricked-and-paved downtowns -- where the devices will measure temperature every few minutes for the next four years. Snyder and Twine hope their results will show in detail where the metro area's warmest and coolest spots are, and perhaps why. They also hope to reveal the dynamics of urban heat in ways only guessed at until now -- tracking, for example, the differences between the temperatures of surfaces and the temperatures of air, and the southeastward drift of urban heat on prevailing winds.

Beyond that, they envision their study as pushing city planners, architects and others to find ways to make urban areas cooler, particularly in warm-weather months.

"We have a fairly good idea what causes urban heat islands," Snyder said. "I think the interesting part comes when you put that together with the impacts, which are poorly understood, on the urban ecosystem, energy consumption and economics."

Costs of urban heat

According to the U.S. Environmental Protection Agency, the annual mean air temperature of a city with 1 million people or more can be about 2 to 5 degrees warmer than its surroundings. In the evening, the difference can be as much as 22 degrees. That means higher energy demand and utility bills for air conditioning, greater threats to the health of people unable to escape the heat, and increased risks to aquatic life due to warmed-up runoff. More than 700 deaths were attributed to the effects of record heat in Chicago in July 1995.

Snyder and Twine, whose project is funded by the University of Minnesota's Institute on the Environment, aren't limiting their research to the Twin Cities. They want to use satellite data to develop fine-grained pictures of heat in 100 cities around the world, in hopes of being able to determine the effects of landscape, land use, transportation, industry and other factors on temperature.

Cities occupy a small share of the landscape, "but we're concerned with them because that's where most people live," Twine said.

Heat waves and other phenomena "will continue to occur," Twine noted, "but when they occur in urban areas that already have higher temperatures, it can exacerbate the conditions."

The next step will be for policymakers and builders to figure out how to reduce urban heat. In the Twin Cities, vegetated "green" roofs at Target Center, Central Library and Minneapolis City Hall are major examples of efforts to convert what could be large, heat-absorbing surfaces into cooler ones. But there are other ideas out there that could change the way cities look.

White rooftops

Looking out the window of her Oakland, Calif., office recently, Lisa Gartland, a heat island mitigation consultant, could see white rooftops, now required under California law to reflect heat, rather than absorb it, ideally reducing air-conditioning needs. She has also been trying to persuade developers and even road builders that asphalt need not be black. With pigment, it can even be white, she said.

"Part of it is changing people's ideas of what a city should look like," Gartland said. "Lots of education is needed -- most of it at the level of contractors, municipalities and architects. "

The city of Sacramento realized a savings of $26 million in 1997 through cooling techniques -- mostly from tree planting, said city planner Erik DeKok. More recently the city has required that 50 percent of all parking lot surfaces be shaded. DeKok said developers have taken that requirement beyond trees and have been furnishing parking lots with carports, whose roofs in turn are covered with energy-generating solar cells.

Grass, trees and vines may be the simplest materials for reducing urban heat, Gartland said. Phoenix is one city that is cooler than its surroundings because people have planted so much greenery in what had been a desert, Gartland said. That isn't a panacea there, however, because the city also has water-use issues.

In the Twin Cities, residents might want to hang on to every degree they can in the winter, Snyder acknowledged. And Kathy Klink, a University of Minnesota geography professor who has studied urban heat in the winter, found in a recent study that Minneapolis in winter is already slightly more than 1 degree warmer than surrounding areas, because of a combination of heat from buildings and traffic, as well as plowed pavement.

But heat mitigation strategies would be unlikely to cool the cities in the winter, Klink said. Short days and reflective snow cover both limit the amount of radiation the Twin Cities might absorb and would probably overwhelm any heat-reduction efforts.

Urban American Gothic

Snyder and Klink are relying on volunteer property owners to accept a thermometer, from which students will periodically download data for the next four years. About 50 have been placed so far.

Snyder noted that a digital, urban temperature-reading network is likely to have different demographics from the far-flung collections of farmers and stay-at-homes in the standard picture of weather observers. In fact, the project's title, "Island in the Sun," was "loosely borrowed" from that of a song by the alternative rock group Weezer, Snyder noted. Twine added that being home to jot down numbers every day -- after milking cows, presumably -- won't be a requirement.

"People are busy. But this is easy. You just leave it alone," she said.

Information on volunteering for Snyder's and Twine's project is available at

Bill McAuliffe • 612-673-7646