10 July 2014 — BRIEF: A city’s local background climate may have a greater role than previously thought in variations in the urban heat island effect — a phenomenon that causes many cities to be warmer than their rural surroundings.
The UHI effect has long been thought to be controlled by the lower rates of evaporative cooling in urban areas, which should tend to increase temperatures, but in a report published in Nature this week, Xuhui Lee and colleagues show that UHI variations can instead be explained by changes in the efficiency of convection between the land surface and the lower atmosphere.
If an urban area is aerodynamically smoother than surrounding rural areas it is harder for heat to disperse into the broader atmosphere and urban warming occurs; conversely, cooling tends to occur in urban areas that are aerodynamically rougher.
“The ‘rougher’ surfaces of the vegetation triggers turbulence, and turbulence removes heat from the surface to the atmosphere,” lead author Lei Zhao said. “But where there is a smoother surface, there is less convection and the heat will be trapped in the surface.”
This convection effect varies with local climate, leading to warming in humid climates and cooling in drier climates.
“There is a synergistic relationship between climate conditions and the urban heat island,” Lee said. “This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity.
“This is a huge concern from a public health perspective.”
In cities such as Atlanta, Georgia, and Nashville, Tennessee, impaired “convective efficiency” has contributed a 3-degree Celsius rise in average daytime temperatures.
However, the research found that it was heat stored in human-built structures that was the dominant contributor to UHI during the nighttime.
The authors suggested that the relationship between the UHI effect and a city’s background climate could aid efforts to mitigate the effects of urban heatwave stress on human health.