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.

Read more.

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  1. The discovery that humid places or cities are worse to live than in dry ones is not new, as you can see in the theoretical study by Sartori (2013) “Evaporation and the planet”, at https://sartori-globalwarming.blogspot.com, where you will also see a mathematical comparison between the thermal behaviors of the Amazon and the Sahara, which shows, among many other things, that the Amazonian atmosphere is warmer than the Sahara one.

    Also, the statement that the buildings must be taller is erroneous. This you can also see through the theoretical study by Sartori at https://escalonamento.tripod.com, where you will know that what makes the wind to circulate more, and thus the higher removal of heat, is the DIFFERENCE of building heights and sizes, not their heights.