NASA is using its latest technology to evaluate the impact of urban forests on the heating of cities. A NASA airborne thermal scanner is able to quickly take a "snapshot" of surface temperatures across the city and identify the "hottest" surfaces within the city.
The additional heating of the air over the city is the result of the replacement of naturally vegetated surfaces with those composed of asphalt, concrete, rooftops and other man-made materials. The temperatures of these artificial surfaces can be 20 to 40 °C higher than vegetated surfaces. Materials such as asphalt store much of the sun's energy and remains hot long after sunset. This produces a dome of elevated air temperatures 5 to 8 °C greater over the city, compared to the air temperatures over adjacent rural areas. This effect is called the "urban heat island".
It would be difficult to take enough temperature measurements over a large city area to characterize the temperature variability, but airborne scanners are ideal for the task. In a study funded by NASA's Marshall Space Flight Center, a series of flights over Huntsville AL. were performed during September 1994 during the period of maximum heating (solar noon) and shortly after sundown when the heat storage is the greatest. From the pictures it is easy to see that the heating of road and parking lot surfaces, especially those using asphalt contribute most to the heating.
What roles do trees play in the "urban heat island" phenomenon? Basically there are two important roles tree canopies play:
An example of the kind of "data" obtained from the NASA scanner is an image from the area around Madison Square Mall (photo) in Huntsville. Warmer temperatures are represented by lighter shades of gray & white and the darker shades as the cooler temperatures. The average temperature was 45.1 °C compared with a nearby forested area at 29.6 °C. A spot check of day temperatures around the Mall shows that in the middle of the parking lot temperatures reach about 48.8 °C . However, a "tree island", a small planter containing a couple of trees in the parking lot is only 31.6 °C. So, even a small area of tree coverage surrounded by a very hot parking lot reduced temperatures by a significant 17.2 °C !!
At night a spot check of temperatures approximately 5 hours after sunset revel that the Mall parking lot is about 24.0 °C and 18.1 °C for the tree island. A nearby forest was 17.1 °C . So the amount of heat stored by the asphalt parking lot was significant. The beneficial effect of the "tree island" is also evident in reducing the storage of heat for an asphalt surface.
Also participating in the project were 13 Huntsville area schools (K-12) with about 250 students. Students took air temperature measurements from a variety of environments in conjunction with the over flights. This provided an excellent educational opportunity for students in understanding the importance of trees in moderating their environment, especially when they had to stand out in the hot sun to take temperature measurements!
The airborne data allow us to quantify the effect of tree canopy cover on the heating of the urban environment. These data provide a foundation for determining a cost benefit of planting trees and to reinforce the need to maintain and develop urban forests. Better design of parking lots to include "tree islands" to shade the asphalt. Tree-lined streets would also shade the concrete and asphalt. Additional benefits would come from shading roofs and reducing the heat load on houses and buildings, thus reducing power requirements for cooling. So think cool and plant a tree.
Funding for this project is gratefully acknowledged from the Marshall Space Flight Centers, Center Directors Discretionary Fund.
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Responsible Official: Dr. James E. Arnold (jim.arnold@msfc.nasa.gov)
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