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Heat Islands
September 07, 2000

Anyone who lives or works in a city knows that you can beat the heat by leaving town. In fact, it’s no secret that cities not only feel hotter than their surroundings, they are hotter.

Now new research shows that some cities are actually changing their own weather, causing storms to pop out of nowhere in some cases, and breaking them up in others. With the help of some high tech tools, scientists are getting better at understanding how sprawling cities turn into urban heat islands.

An island is formed

Urban heat islands form when natural vegetation is replaced by buildings and concrete, causing a region to behave much differently in terms of reflecting and absorbing heat. When intense light shines upon these manmade structures, the surfaces heat up and store the heat throughout the day, and then re-release it approximately two to three hours after sunset. The earliest account of this effect was in 1820 in London, but it wasn’t until the 1996 Olympics in Atlanta that scientists really began to pay attention to it. By analyzing data collected from meteorological towers set up for the games and using computer models, researchers found that storms were developing over Atlanta at odd times.

This animation--generated from satellite data of temperature in Atlanta--shows how the city’s intense heat island can end up creating its own storms.

If you prefer to view it with RealPlayer, click here.

courtesy NASA Marshall Space Flight Center and Qinglu Lin.

Researchers from universities and laboratories across the U.S. then teamed up on Project ATLANTA (Atlanta Land-Use Analysis: Temperature and Air-Quality) to study how the city’s rapid growth might be affecting the climate and air quality. Satellite data show that between 1973 and 1992, about 380,000 acres of forest land disappeared in Atlanta, while acres of single family residential houses increased by 130,000. Using instruments such as ATLAS (Airborne Terrestrial and Land Acquisition Sensor—similar to the kind used on interplanetary space probes) the team was able to see temperatures in 15 different wavelengths.

They found that at times when the air temperature might be 80 degrees Fahrenheit, the surface of the city—particularly the rooftops—was 120 degrees. Such extreme temperatures can have surprising effects. "One of the most fascinating things we’ve found is that cities generate their own weather. And this is something we just found out early this year through analyzing weather satellite data," says Dale Quattrochi, senior researcher at the Global Hydrology and Climate Center, part of NASA’s Marshall Space Flight Center in Huntsville, Alabama. "Ultimately what happens is this releasing of heat causes air temperatures to rise over the city, as opposed or compared to the surrounding rural area, and so you have this bubble or dome of elevated air temperatures over the city which is referred to as the urban heat island effect."

"An urban heat island acts to produce its own rain because the warmth of the city creates low pressure," says Robert Bornstein, professor of meteorology at San Jose State University. "That causes the air to flow into the city from all directions, and that air has no place to go but upward. And any time you can get air rising you are going to initiate a thunderstorm."

The teams have studied other cities as well, including Baton Rouge, Salt Lake City and Sacramento. Next they plan to take a look at Houston, Birmingham, Nashville and Chicago. "The question is how does the geography affect the magnitude of the city and the extent of the heat island itself," says Quattrochi. "We know for example that this urban heat island effect exists in very small cities and the question is: Is there a threshold size?"

Other urban landscape effects

While the urban heat island effect means additional rain in cities like Atlanta, it can also have a different effect on precipitation. In New York City, for example, tall buildings sometimes can push the rain around. "I think cities split approaching rainstorms the same way that water flows around a rock in a stream," says Bornstein. "The winds and the rain are forced around the city because of the physical barrier of the buildings of the city."

Aerial image of Atlanta showing its hottest spots
This aerial image of the Atlanta area shows its hottest spots (in red).
image: NASA

Urban heat islands also affect air quality, because as temperatures increase, so does smog. Smog concentration is usually measured in terms of ozone, an air pollutant that is odorless and colorless but irritating to the respiratory system. Ozone comes from a variety of sources ranging from gas station pumps to smokestacks to car exhaust, and increases as the temperature goes up.

In Los Angeles, for example, for every degree the temperature rises above 70 degrees Fahrenheit, smog goes up by three percent, according to the Heat Island Group at Lawrence Berkeley Laboratory. Cooling the city by just five degrees would dramatically affect smog levels. In Atlanta, Quattrochi says that such a decrease would lower ozone by ten to 14 percent.

Urban heat islands also affects air quality, because buildings and asphalt paving that absorb heat mean more air conditioning and electricity use.

Cooling off

The effects of urban heat islands are sure to increase as more and more people flock to cities (predictions call for 80 percent of the world’s population living in cities by 2025), so the heat is on to find ways to cool them. Planting trees in strategic locations around cities is one way of offsetting the effects of urban heat islands. Another strategy would be to use light-colored surfaces for roofs and roadways that would reflect heat instead of absorb it. Concrete is significantly cooler than asphalt, Quattrochi points out. "There are new surfacing materials for asphalt that actually make it a little bit lighter so it doesn’t heat up as much and is again fairly cool in comparison to other surfaces," he says. Such steps would hopefully bring energy use down as well.

Also, the new technology allows researchers to generate color maps showing hot spots and cool spots. Government officials and urban planners can then use the maps to plan future urban growth.

Elsewhere on the web:

Urban Heat Islands Pilot Project

How Roof Heat is Transferred

Cool Roofing Samples

Urban Heat Islands in Japan

Urban Heat Islands in Australia

Urban Heat Islands Mitigation

by Jill Max

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