Cities Make Own Weather Due to Trapped Heat, Expert Says

John Roach
for National Geographic News
January 17, 2006
During winter storms many city folk may praise warmer downtown
temperatures for keeping the streets snow and ice free.

But urbanites ought to take steps to curb this phenomenon before localized temperature differences become a global weather problem, a meteorology expert says.

Tightly packed streets, parking lots, concrete buildings, and dark roofs absorb sunlight all day, explained Dale Quattrochi, a geographer with the NASA Marshall Space Flight Center in Huntsville, Alabama.

Quattrochi says that the absorbed sunshine keeps cities 1° to 10°F (0.56° to 5.6°C) warmer than the surrounding countryside.

When Earth is viewed from space with equipment that maps surface temperature, urban areas appear as islands of heat.

Quattrochi speaks about the urban heat-island effect in a Pulse of the Planet radio broadcast today. (The radio program and this news story are partially sponsored by the National Science Foundation.)

In winter the extra warmth may be just enough to prevent snow from sticking to the streets, even when the air temperature hovers near freezing.

But in summer the effect can bump up energy costs due to air conditioners and can even cause cities to create their own weather, Quattrochi said.


According to Quattrochi, city streets and rooftops pump warm air into the lower atmosphere long after the surrounding countryside has cooled. This is called the chimney effect.

"When hot air rises, it condenses—it forms clouds," Quattrochi said.

"[The chimney effect] is enhanced through a period of time, and as it keeps pumping hot air into the atmosphere, clouds keep building and building, and pretty soon it starts raining."

The rain falls on and downwind of cities.

More and larger cities would mean more rain. The United Nations estimates that 60 percent of the world's population will be city dwellers by 2025.

As city populations continue to grow, the localized weather-making ability of cities may begin to affect global weather.

What's more, the extra summertime heat creates greater demand on energy resources for cooling sunbaked city dwellers.

To lessen the heat-island effect, the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy are urging cities to plant more trees, use reflective roofing materials, and grow rooftop gardens.

Cool Trees

Trees are city planners' first line of defense in the fight against the urban heat-island effect, Quattrochi said. But to be effective, planners must select the right kinds of trees, such as oaks and maples, and plant them in the right places.

Oaks and maples are big, leafy trees that provide shade, use the sun's energy to grow, and give off water vapor that cools the air through evaporation, he explained.

Other trees, such as pine and eucalyptus, naturally emit chemical compounds that contribute to big-city problems, such as ground-level ozone and smog.

To get the most benefit from trees, Quattrochi said, planners should plant in areas that are currently devoid of vegetation, such as shopping mall parking lots and crowded city streets.

Also, planting trees on the south and west sides of a home or building will help keep the interior cool during the hottest part of the day, he added, reducing cooling costs.

According to the EPA, more trees in cities can improve air quality, reduce carbon dioxide emissions, decrease storm-water runoff, and improve community livability.

"Everybody likes trees," Quattrochi said. "They add to the overall ambience of the city. I think urban planners look at planting trees now not only as, This is pretty, but, It's effective, it's cost effective."

Bright Roofs

Another way cities can combat urban heating is with reflective roofing materials. Rooftops are predominately dark colored, which means they readily absorb heat, Quattrochi explained.

For example, a recent survey of summertime temperatures on rooftops in Atlanta, Georgia, revealed one building roof that was 170°F (77°C).

"What happens is you have an air conditioner on the roof that is trying to suck in and chill down 170° air," Quattrochi said. "To chill it, the air conditioner has to work harder, and cooling costs go literally through the roof."

Government agencies are working with several manufacturers to develop more energy-efficient roofing materials, such as smooth, bright-white surfaces that reflect 60 to 80 percent of the incoming sunlight.

An EPA study of ten buildings in Florida and California showed that use of cool-roof materials saved residents and building owners 20 to 70 percent in their annual cooling-energy use.

Another possible solution is to plant a roof garden, which on a hot summer day can make the rooftop cooler than the air. Other benefits of this approach include a reduction in air and water pollution and increased habitat for birds.

The idea has already taken root in Chicago, Illinois, where more than 150 roof gardens are already completed or under construction, including one on city hall.

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