Making and Breaking of Heat Islands

On May 11-12, 1997, NASA used a specially outfitted Lear Jet to collect thermal data on metropolitan Atlanta, Georgia. Nicknamed “Hot-Lanta” by some of its residents, the city saw daytime air temperatures of only about 26.7 degrees Celsius (80 degrees Fahrenheit) on those days, but some of its surface temperatures soared to 47.8 degrees Celsius (118 degrees Fahrenheit). In this image, blue shows cool temperatures and red shows warm temperatures. Pockets of especially hot temperatures appear in white.

One trend is an increase in the number of city dwellers One trend is an increase in the number of city dwellers. “About half the world’s population—3 billion people—now live in cities. In a couple decades, it’s going to be 5 billion people The second trend is Global Climate Change

Contributors to Heat Islands Stone, brick, concrete, and asphalt surfaces that are impervious to water can trap the Sun’s heat. Not only do cities concentrate such heat-trapping surfaces, they also contain other sources of heat, such as vehicle motors and smokestacks.

August 12, 2002 (light colors hottest) The team studied the city as a whole, as well as six “hotspot” areas—including parts of Manhattan, the Bronx, Queens, and Brooklyn—where air temperatures near the ground were higher than the city-wide average. Each area was serviced by Con Edison, the local power company, so the scientists could compare electricity use. Each area also had available space so that the mitigation strategies the team considered could be modeled in the study and potentially implemented later on.

Gaffin and his colleagues used Landsat data to assess New York City’s summer heat. The right map shows temperature, with cooler temperatures appearing in blue and hotter temperatures appearing in yellow. The left image shows vegetation, with beige indicating sparse vegetation and dark green indicating dense vegetation. The maps show a correlation between dense vegetation and cool temperatures, and between sparse vegetation and high temperatures. NASA’s Landsat satellite captured these images of New York City on August 14, 2002, at 10:30 a.m.

Electricity demands and temperature Electricity demand is highly sensitive to temperature. This graph shows demand (in gigawatt hours) versus temperature (in degrees Fahrenheit) in New York State in the late 1990s. A gigawatt hour is a unit of power equal to 1 billion watts. Electricity demand is slightly lower on weekends and holidays (blue dots) than on weekdays (red dots). Demand increases sharply as temperatures rise above about 60 degrees.

Breaking Heat Islands Green roofs can range in complexity from a shallow layer of soil and plants to more elaborate rooftop garden with trees and shrubs. The more elaborate roofs place greater structural loads on buildings and require more maintenance, so planning and implementing them is more complicated. This photo shows the garden-like green roof atop the Solaire in Battery Park City in New York.

Standard Roof vs. Green Roof During the Penn State 2003 study, Gaffin and his collaborators measured the temperatures on both green and dark roofs. Both kinds of roofs warmed during the day and cooled overnight. While dark roofs cooled slightly more overnight, however, they warmed up much more during the day than their green counterparts. At their warmest, the dark roofs reached roughly 70 degrees Celsius, whereas the green roofs only reached about 40 degrees.

Although longer-lived than a standard roof, a vegetation-covered roof is more complex, and its components mitigate both urban heat and urban runoff. Water that would otherwise reach a storm sewer is trapped by the green roof’s soil and vegetation.