Urban islands

Heat island effect is no vacation

<br> The green roof of this Coralville, Iowa, park building is growing strong.

The green roof of this Coralville, Iowa, park building is growing strong.

Photo courtesy of U.s. Department of agriculture

Sustainable Space columnists Lori Brown and Greg Kallio are professors in the College of Engineering, Computer Science and Construction Management at Chico State University

When you hear the words “heat-island effect,” does the thought of relaxing on a beach in Oahu, sipping a mai tai, come to mind? Chillin’ on a beautiful Hawaiian island, not worrying about the daily grind of work, is an island effect we all could use. To the more ecollectual sorts, stressed out about the impact our cities are having on the environment, the term “heat island” describes built-up urban areas that are hotter than nearby rural areas.

The urban heat-island effect is generated by large expanses of buildings, asphalt and other human-made conditions that alter landscape that would otherwise be covered by fields, forests and bodies of water. Consider the city of Chico: It wasn’t all that long ago that it was primarily open land and vegetation. Developing roads, buildings and other infrastructure replaced these surfaces, leading to the formation of urban heat islands. According to climate experts, cities should try to reduce this effect.

Too hot
Heat islands occur when the sun heats exposed urban surfaces, such as roofs and pavement, to temperatures between 50 and 90 degrees hotter than the air, while shaded or moist surfaces, often in more rural areas, remain close to air temperatures. The annual mean air temperature of a city with 1 million people or more can be 1.8 to 5.4 degrees warmer than its surroundings. In the evening, the difference can be as high as 22 degrees. The increased heat of areas within the city creates additional discomfort, requires more energy for cooling buildings and homes, and increases pollution.

Increased heat enhances photochemical reactions, which increases the particles in the air and thus contributes to the formation of smog and clouds. To get a first-hand feel for heat-island effect, try standing in the middle of an expansive parking lot—like the one at Chico’s Costco—between noon and 4 p.m. sometime this summer. You’ll melt.

Cooling trends
The good news is that communities can combat the negative impacts caused by urban heat-island effect by increasing tree and vegetative cover, replacing conventional roofs with green varieties or installing cool reflective roofs, and using cool pavements.

Trees and vegetation provide shade, which helps to lower surface temperatures. Air temperatures are reduced when plants release water to the surrounding air, through a process called evapotranspiration, helping to dissipate ambient heat. Urban areas typically covered by dry, impervious surfaces, such as conventional roofs, sidewalks, roads and parking lots, can be replaced with more eco-friendly urban materials developed to be more solar-energy reflective and less absorptive than conventional materials.

Radical roofing
The term “cool roof” is used to describe roofing materials that reflect a large portion of the sun’s energy. Cool roofs also may have a high thermal emittance and release a large percentage of absorbed heat. Cool materials reduce the absorption of solar heat and the transfer of this heat to the surroundings. Lighter-colored roofing materials come in shades of white, beige, light gray and terra cotta. When replacing or installing a new roof, how about going with a live or green roof? Green roofs significantly reduce heat-island effect and can help lower a building’s energy cost for cooling.

Underfoot alternatives
Another option for reducing heat-island effect is using permeable pavements, which can be constructed from a number of materials, including concrete and asphalt (filled with soil, gravel and grass). Water-pervious materials such as gravel, crushed stone, open paving blocks or pervious paving blocks for driveways, parking areas, walkways and patios all help to reduce urban heat-island effect.

Chico’s sizzling summer heat isn’t going to go away anytime soon, but the city’s heat-island effect can be reduced by improving one surface at a time.