Cities are often hotspots of, well, heat. As cities sprawl and heat waves worsen, the urban heat island effect is subjecting city dwellers to the dangers of heat stroke, not to mention thirstiness, panting, and unsightly perspiration patterns.
One problem is that concrete and mortar store more heat than do soils and plants. Running air conditioners keeps rooms in cities cool, but it also heats up the air outside. And there are differences in the efficiency with which different landscapes convect heat into the lower atmosphere.
Understanding the precise roles that these factors play in the broiling of cities could help planners sculpt urban landscapes that more quickly shed themselves of excess heat.
Zhao's main conclusion was a discouraging one—the single biggest factor influencing the heat island effect is the amount of moisture in the air.
Lei Zhao is a doctoral candidate at Yale University who has spent five years investigating the factors that influence this effect. "It's generally a hot topic," he says. "People have been talking about urban heat islands for a long time. Through my readings of the literature, I found a number of studies talking about the different factors that contribute. But it was still unclear which ones contribute how much to the problem."
For his Ph.D. project, the results of which were published online as a letter Wednesday in Nature, Zhao analyzed satellite data from 65 American and Canadian cities, comparing urban temperatures with those in surrounding areas. He ran simulations of the urban heat island effect in each of those 65 cities using the National Center for Atmospheric Research's Community Earth System Model. That helped isolate and pinpoint the heat-trapping and heat-shedding roles played by cities' physical features.
Zhao's main conclusion was a discouraging one—the single biggest factor influencing the heat island effect is the amount of moisture in the air. The wetter and more humid the climate in which a city was built, the more heat that city will trap compared to the surrounding landscape. "A city's morphology has a less important role than its background climate," he says. That means cities like New Orleans and New York face bigger heat-island challenges than do Phoenix or Las Vegas.
But Zhao found two important factors that can be influenced through urban design.
The "smoothness" of a city compared to its surrounding land can strengthen the urban heat island effect. Rougher landscapes are more efficient heat convectors, meaning a jungle of skyscrapers surrounded by grasslands would stay cooler than a bunch of low-lying and similarly shaped townhouses flanked by forests.
"If I increase the roughness of a city, that will make my city a more efficient heat convector," Zhao says. "But this is hard."
A less powerful effect, but one that's easier to influence, comes from the reflectiveness (also known as albedo) of building surfaces. White roofs, like the ones increasingly being implemented in Chicago, help bounce heat back into the atmosphere.
"Increasing the albedo is a more practical way to convect the heat," Zhao says. "According to our results, increasing the city albedo can mitigate the urban heat island effect in both daytime and nighttime."