In urban areas, wherever buildings replace vegetation, their heat-absorbing concrete, steel and asphalt can make a city 5 to 7 degrees hotter than the surrounding countryside. Hi, I’m Bryan Yeaton for The Weather Notebook’s Climate Change series.

To combat this “heat island effect,” some planners suggest planting vegetation way up there. These rooftop arboretums— known as “eco-roofs” or “green roofs” — can change building tops from barren patches as hot as any desert, to cool, moist oases. An eco-roof can reduce extreme summer temperatures on a roof surface by up to 100 degrees Fahrenheit.

Feasibility studies estimate that greening the rooftops on all of Chicago’s city buildings could save $100 million annually in energy costs. A 3 to 7 degree drop in average roof temperature, for example, reduces a building’s air-conditioning needs by 10 percent, and cuts energy usage by 30 percent. Eco-roofs also act as natural sponges during rainstorms. A roof with 4-5 inches of soil will hold about an inch of rainwater, and when that moisture evaporates, it further cools the roof and surrounding air.

Several major US cities including New York and Portland, Oregon are considering eco-roof designs. Thirty-two percent of Manhattan’s land area is covered by roofs, the equivalent of four-and-a-half Central Parks. Chicago’s City Hall and Seattle's City Hall and Justice Center already have rooftop gardens. Even industry is benefiting: the Ford Motor Company has installed a 10.4-acre green roof atop its assembly plant in Dearborn, Michigan.

Europe has seen more than 100 million square feet of roofs planted since the 1970s. In Germany, seven percent of all new construction have eco-roofs.

Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is supported by Subaru of America, with additional help from Environmental Defense.

Today's Links

Is that a daisy growing on your roof?
http://www.christiansciencemonitor.com/2003/0409/p19s01-liga.html

Green Roofs for Healthy Cities
http://www.greenroofs.org