Cities Team With NASA And EPA For "Urban Forests" Study

May 01, 1998

A highly successful experiment in measuring how cities can keep their cool will be repeated in several U.S. cities this summer.

The principle behind the Urban Heat Island Pilot Project (UHIPP) relates to the differences in cooling and heating between the natural and manmade surfaces that make up a city.

"Urban forests are important to keeping cities cool," said principal investigator Dr. Jeff Luvall. "What's important are both the extent and arrangement of these forests."

The Global Hydrology and Climate Center (GHCC) in Huntsville, Ala., working with the U.S. Environmental Protection Agency and several local governments, will conduct the Urban Heat Island Experiment. The GHCC is a joint venture by NASA's Marshall Space Flight Center, the Universities Space Research Association, and the Space Science and Technology Alliance of the State of Alabama. UHIPP follows the successful Urban Heat Island Experiment in Atlanta in May 1998.

"While UHIPP is quite complex," Luvall explained, "at its core is the fact that the evaporation of water absorbs a lot of heat. Plants, and trees in particular, evaporate large amounts of water from their leaves. The energy required to evaporate water is taken from the air and from the sunlight intercepted by the leaves, thus cooling the air. Trees are also very effective in shading the ground, thus preventing the heating of the surface by sunlight."

On the other hand, asphalt, concrete, and other manmade materials are very effective at absorbing light and reradiating it as infrared radiation that raises the temperature of the air. In turn, that makes air conditioning systems work harder, even after sunset. As a result, utility bills go up and heat stress as a result of increased heat over the city can affect human health.

Inframetrics thermal infrared camera (which will not be used in the experiment) shows how trees make a difference in the middle of a parking lot. The image at left was taken during the day, while at night (right half) the shadow of the tree is still visible.

Atop all that, depending on meteorological conditions, the hot air will form a dome or bubble over the city which is several degrees warmer that the surrounding countryside

ATLAS lets scientists see how different objects reflect or absorb and emit heat. It's related to the greenhouse effect that made life possible on Earth: incoming sunlight passes through the atmosphere and is absorbed at the ground. The now-warm objects reradiate this energy at longer wavelengths - thermal infrared radiation - that the atmosphere absorbs rather than passing back into space.

What scientists see, through ATLAS and other instruments, is that cities follow this rule to the extreme. Asphalt parking lots and roofs soak up virtually all of the radiation that falls on them and reradiate it as heat. Although the heat island effect has been known for a long time, little has been done to measure it in a quantifiable way that governments, decision-makers, and people in general can use in planning cities. Remedial measures can include planting trees or roofing buildings that reflect rather than absorb light.

The experiment will comprise measurements with ATLAS and a 9x9-inch film camera aboard a NASA Lear 23 jet (right) from Stennis Space Center, instruments aboard polar and geostationary orbit satellites, and scientists and assistants with instruments on the ground. ATLAS will be used over Baton Rouge, Sacramento, and Salt Lake City.

Like Atlanta, site of NASA's 1997 heat island experiment, these cities have experienced rapid growth in recent years. Their lessons could help guide cities in other nations, too.

"Developing countries are expanding so rapidly that you have to ask how they affect local and regional weather and, possibly even global, climate," said Dr. Dale Quattrochi, a co-investigator on this project and principal investigator for the Atlanta experiment.

Updates on plans and activities will be posted here and at the GHCC as the Urban Heat Island Pilot Project (UHIPP) develops.

Because the current weather patterns affecting Sacramento and Salt Lake City will not change in time for at least the Sacramento flight, the science team has delayed the western flights to see if the weather patterns return to a "normal" year.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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