The Urban Heat Island: Microclimate Warming
By Diana Clarke
In the summer, I often visit a park in Sacramento County because it is cooler than surrounding areas. I didn’t know why it was cooler even in direct sunlight until I visited the Sacramento Tree Foundation at http://www.sactree.com/.
I discovered that trees not only shade us from the sun, they also reduce ambient temperature. Trees transpire water which cools the air by taking heat from the air to evaporate the water. Trees also filter pollution and reflect solar radiation.
Heat Island Effect
In areas like downtown Sacramento, which has fewer trees than the surrounding communities, the ambient temperature is higher. Structures, such as dark colored rooftops, concrete, and asphalt also contribute to the “heat island effect.” The Environmental Protection Agency (EPA) describes the heat island effect as “urban and suburban temperatures that are 2 to 10 degrees Fahrenheit hotter than nearby rural areas.”
The image was taken Monday, June 29, at 1 p.m. local time by the ATLAS imager aboard a NASA Lear 23 jet equipped with various sensors and cameras for UHIPP.
From surface temperature estimates, the white areas are about 60 degrees C (140 degrees F), said Dr. Jeff Luvall, the principal investigator at NASA's Marshall Space Flight Center. Dark areas (vegetation) are approximately 29 to 36 degrees C (85-96 degrees F). Since the image has not been calibrated, absolute temperatures will change after calibration, but the relative temperature differences between surface types will not.
Dark-colored surfaces have a low albedo whereas trees and light- colored surfaces have a high albedo. Albedo stands for the surface’s ability to reflect rather than absorb solar radiation.
Rooftops, asphalt and concrete absorb solar energy, raising surface temperature 50-70 degrees Fahrenheit hotter than the ambient temperature. These structures reflect, store and release heat into the air even after dark. Dark-colored rooftops have been shown to heat up to as much as 170 degrees Fahrenheit. On the other hand, white rooftops can reflect 65% solar radiation, providing a cooling effect.
Using an infrared thermometer called a heat spy, Ray Thetheway, Sacramento City Councilman and Executive Director of Tree Foundation in Sacramento, showed the surface temperature (not the ambient temperature) difference between a white car and a black car. The white car was 87 degrees Fahrenheit whereas the black car was 134 degrees Fahrenheit. Without air conditioning, which one would you prefer riding in? The same goes for homes and office buildings. When more air conditioning is required, more energy is consumed.
Sacramento Tree Survey
On Saturdays between June 23 and July 21, 2007,
the Tree Foundation and volunteers will conduct a tree census within the 479 square miles of the Sacramento area. The size, number, species, and health of the trees will be determined and used to help plan for a
healthy urban forest that would help lower the region's temperature and also create shade, diminish air pollution and capture storm water.
“This is the largest project of its kind in the nation," says Cara Smith of the Sacramento Tree Foundation."
The benefits of Trees
Adding more trees to parking lots and city streets would:
- reduce air pollution.
- help lower the air temperature by cooling the air and shading the asphalt and concrete.
- reduce energy consumption.
- provide habitats for birds and other creatures.
- add natural beauty to the urban landscape.
- help reduce noise.
- patially block UV light
- remove and store C02 from the atmosphere.
"Mature trees may store 1,000 times more carbon.
The U.S. Forest Service estimates that carbon storage by urban forests is between 400 and 900 million metric tons nationally." (EPA)
You can view Urban Heat Island Effect video, courtesy of the Environmental Protection Agency and the Weather Channel.
Sources:
“Heat Island”, Urban Climatology and Air Quality, NASA/Marshall Space Flight Center and
Global Hydrology and Climate Center, August 5, 1999
http://wwwghcc.msfc.nasa.gov/urban/
Sacramento Tree Foundation
http://www.sactree.com
“Sacramento Glows with Urban Heat,” Dave Dooling, Science @NASA, July 1, 1998,
http://science.nasa.gov/newhome/headlines/essd01jul98%5F1.htm
“Gimme Swelter,” Sacramento News and Review, Sarah Felts, June 5, 2003
“Heat Island Effect,”Environmental Protection Agency
http://www.epa.gov/heatisland/
“Heat Island Effect”: Sacramento, Environmental Protection Agency, May 18, 2005
http://www.epa.gov/heatisland/pilot/sacramento.html
Images
Photo Credit: NASA/Marshall Space Flight Center and
Global Hydrology and Climate Center
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Last modified: 07/08/07
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