Urbanites around the world know that summer in the city can be brutally hot, but for years the reason why urban areas are notably warmer than their surrounding rural areas was unclear. In a new study published in Nature, scientists tackled the issue of Urban Heat Islands (UHI), as this phenomenon is called. They examined the land surface temperatures and vegetation cover of 65 cities in the United States and Canada. They concluded that although loss of vegetation and heat storing structures contribute to urban warming, inefficient convection, or how much heat is released back into the lower atmosphere, is the primary cause of Urban Heat Islands.

In a city's surrounding countryside, natural surfaces, like rocks and trees, trigger air turbulence, which moves hot air from the surface and brings in fresh, cool air. The study determined that the smooth surfaces of buildings, pavement, and other man-made structures in the city dampen air turbulence and trap heat on the surface. This is especially true for cities with a more humid climate, particularly in the Southeast United States. These cities are often surrounded by dense vegetation, where the rough canopy of treetops effectively dissipates heat. In comparison to miles of forest, smooth concrete rooftops amplify the UHI effect and cause a 5-degree rise in average daily temperatures, according to the study.

Xuhui Lee, a Yale professor and one of the study's authors, said in a statement that increasing temperatures from climate change will only exacerbate the UHI effect. Lee said that as more and more people move into cities, the UHI effect could have significant impacts on human health. Indeed, heat waves are already listed by the U.S. Centers for Disease Control and Prevention as the deadliest natural disaster.

According to the study, increasing a city's convection efficiency would require massive changes to the city's architecture, such as lowering building heights. The study suggests that a much more viable solution would be to focus on increasing the city's surface albedo, or how much sunlight it reflects versus absorbs. Minor changes, such as switching to a lighter color for parking lots, roads and roofs, can make a significant impact on the amount of heat absorbed during the day. According to a 2009 paper, Global Cooling: Increasing Worldwide Urban Albedos to Offset CO2, a building can save more than 20 percent of its cooling costs by increasing its rooftop albedo from 10-20 percent to about 60 percent. The potential savings from such a move could lead to savings of more than $1 billion per year in the United States alone.

For more information on the benefits of increasing albedo, check out our July 2011 briefing Cool Roofs for Cooler Summers and our 2012 fact sheet, Cool Roofs.