Urban Shadows

Andrew Smeltz
January 01, 1998

From her window on the sixth floor of Walker Building, Traci Arthur can see wooded mountain ridges and, in the valley below them, the sprawling influence of the University. In her office she analyzes data reflecting the similar urbanization of Chester County. Just west of Philadelphia, Chester County is one of the fastest growing areas in Pennsylvania. On Arthur's maps, development looks like a red finger stretching west through green country-side along U.S. Route 30. The finger's shadow spreads out from the highway: areas of anticipated development.

Land Cover Classification maps

Development, shown in red, can change more than the view: It also changes the local temperature and rainfall, according to a study linking these satellite maps with a climate model.

The data Arthur interprets were collected by two satellites, the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer (AVHRR) and the Landsat Thematic Mapper (Landsat TM), from 1986 through 1995. Both AVHRR and Landsat TM measure how the Earth reflects and emits radiation, allowing Arthur to locate warmer areas. Because different surfaces have different reflective characteristics, says Arthur, she can also tell from these maps which regions are paved over and which are still grass and trees. For instance, she can see rapid urbanization during the ten years of her study along major roads like U.S. Route 30 and around Marsh Creek State Park.

Arthur, a graduate student in meteorology, inputs the satellite data into a surface climate model, developed by her adviser, Toby Carlson. Using the surface climate model, Arthur can see how the microclimate of an area changes as the area becomes more and more developed. She can also predict how future development will affect the climate. Says Carlson, "Urbanization affects that area in terms of rainfall and temperature, but my own feeling is that the effect on the wider climate is negligible." Rainfall and temperature however, are often what make an area liveable.

According to the climate model, urbanization has brought a dramatic temperature increase to Chester County. Temperatures in most areas under development rose 36 percent in ten years, while temperatures in the areas developed around Marsh Creek State Park rose 109 percent.

Developers don t usually think of their actions as affecting the climate, but, as Arthur writes in an article for The Pennsylvania Planner, "With urbanization, concrete and pavement and massive tightly spaced, irregularly shaped buildings replace the original habitat, leading to an urban microclimate." An urban microclimate, she says, usually has higher temperatures and lower moisture.

Currently, urban planners use aerial photography to get a bird's-eye-view of an area, but renting an airplane and hiring a photographer is expensive. So pictures are usually taken only once every few years. Satellite images, however, are a different story. Data from the AVHRR are free, and even $4,000 for images from the Landsat TM can be less expensive than hiring a plane. Perhaps most significant, the images from the satellites are more flexible; not limited to the visual spectrum.

Arthur hopes not only urban planners, but regional planners, county commissioners, zoning boards, and township supervisors will take advantage of her work. She has already presented her research to planners in Chester County, and the planners, hesitant at first, have expressed interest in learning more. Although developers may not care about the climatic effects of urban sprawl, most residents do.

Traci Arthur is a graduate student in the department of meteorology, College of Earth and Mineral Sciences, 601 Walker Building, University Park, PA 16802; 814-865-1678; tarthur@essc.psu.edu. Her adviser is Toby Carlson, Ph.D., professor of meteorology, 619 Walker Building; 863-1582; tnc@essc.psu.edu. For more information visit the project's Web site at http://www.essc.psu.edu/~tnc/urban. This research is funded by NASA's Mission to Planet Earth.

Last Updated January 01, 1998