Southface Home Page Southface home page
Resources and Services> Publications >Journal>Summer 2006 > Our Own Bit of Shade

About Southface
Get Involved
Resources & Services
Programs & Events
Who We Reach

Our Own Bit of Shade

By Southface Staff with thanks to Cheryl Kortmeier of Trees Atlanta for her contribution to this article.

The “dog days” of summer are upon us here in the South and despite the fact that children are heading back to school shortly, Metro Atlantans know that hot temperatures could possibily continue into October before giving in to the crisp cooler days of fall.

What’s more, because the Atlanta metro area’s expansive growth over the past 25 years has created one of the nation’s largest urban heat islands, summer temperatures in our region can be as much as 10°F above our increasingly faraway rural surroundings, as well as several degrees hotter than in past decades.

In a study conducted by NASA's Global Hydrology and Climate Center in Huntsville, AL, researchers state“… urbanization has consumed vast acreages of land adjacent to the city proper and has pushed the rural/urban fringe farther and farther away from the original Atlanta urban core. An enormous transition of land from forest and agriculture to urban land uses has occurred in the Atlanta area in the last 25 years, along with subsequent changes in the land-atmosphere energy balance relationships,” meaning temperature and air quality issues.

In 1960, a mere 12% of homes in the region had air conditioning, an almost unthinkable exclusion for even the most energy conscious of us today. Before the advent of central air conditioning, home cooling was a matter of site and design to take advantage of natural cooling forces. Cooling and heating use more energy than any other system in your home. Air conditioners can consume up to 1/6th of the electricity used in the US and on a hot summer day the total soars to 43% of the U.S. peak power load, according to the U.S. Department of Energy, which also states that HVAC systems emit over half a billion tons of carbon dioxide and 24% of the sulfur dioxide into the atmosphere annually.

As individuals, we can lower both the cost of cooling our homes and the impact we have on the environment by employing passive cooling techniques. One solution that homeowners can take is the planting and protection of shade trees on their property to provide properly placed shade for their homes.

Trees Atlanta, a non-profit citizen’s group dedicated to protecting and improving Atlanta’s urban environment by planting and conserving trees, states that strategically planted trees can reduce summer cooling costs by 30 to 40%. When planting, pay attention to the structural surfaces that receive the most summer heat, typically the western and eastern exposure. The most efficient shading occurs before the sun’s heat reaches the building, which is why the effective positioning of shade trees makes so much sense for natural cooling. By throwing shade on the walls, windows and roof, trees measurably reduce the heat absorption on those surfaces. They can also shade driveways, patios, sidewalks and other hard surfaces that bounce heat onto the structure.

For a combination of summer cooling and passive heating in winter, consider planting broad canopied deciduous trees on the southern, eastern and western exposures. The expansive, dense coverage in summer will provide the highest amount of shade for the otherwise hottest surfaces. Then when the leaves fall in the cooler seasons, the bare limbs allow solar heat to reach the building for added warmth in winter.

In addition to shading, trees have another important cooling function. In order to keep cool itself a tree draws water out of the ground, through its capillary system and onto the surface of its leaves, much like humans sweat to cool off. Called evapotranspiration, this evaporative cooling process cools the surrounding air as well. “ One mature, properly watered shade tree with a crown of 30 feet can evapotranspire up to 40 gallons of water a day, which is like removing all the heat produced in four hours by a small electric space heater.” (The Heat Island Project at Berkeley National Laboratory, 1990)

If you are building, keep in mind that obviously big trees provide more shade that small ones, so take measures to preserves as many existing trees as possible and then plant new trees immediately after construction.

Under the Canopy

On a community-wide scale, trees have great potential for cooling urban areas and cleaning the air. On the other hand, Atlanta’s current pace of tree removal will greatly increase the area’s urban heat island and add to our air quality problems. According to Trees Atlanta, there is 65% less tree cover in metro Atlanta today than in 1972 and more than 50 acres of trees are eliminated daily due to development. While many of us take pride in our parks and still revel in the sight of the Atlanta skyline rising above the trees as we land at Hartsfield Jackson International Airport, the fact of the matter is that the U.S. Forest Service ranks Atlanta last in terms of shaded streets in the downtown area. This places us behind Newark, Chicago, and even Los Angeles – long the poster child for smog, sprawl, and poor intown quality of life among American urban areas.

According to the Environmental Protection Agency, Atlanta today has a tree cover over about 27% of its land area. In order to create a tree canopy capable of eliminating the urban heat island effect and placing Atlanta on the path to cleaner air, we would need to increase our tree cover to a minimum of 40%. As a community, we have yet to slow the process, much less begin to rebuild our tree canopy, but by investing in trees on our own property in a strategic manner, we can begin to turn the tide, at the same time, reducing our own energy use and utility costs.

Resources

Trees Atlanta www.treesatlanta.org 404 522-4097

Cool Communities A program of American Forests www.americanforest.org 202 955-4500

Southface website www.southface.org/publications Sustainable Design, Construction and Land Development, Guidelines for the Southeast

Southface Energy Institute is located at 241 Pine Street NE, Atlanta, Georgia 30308
Phone: (404) 872-3549 Fax: (404) 872-5009