Friday, July 11, 2014

Cities Are Trapping More Heat, and Even Planting More Trees Won't Cool Them Down


They're planting trees in Louisville, Kentucky, and they're not for making baseball bats. They're for fighting back against the summer in one of the fastest growing "urban heat islands"  in the United States. 

But according to the researchers behind a study just published in Nature, if Louisville wants the plan to work, it's pretty much going to need to plant a downtown forest.

As the US Environmental Protection Agency explains “the term 'heat island' describes built up areas that are hotter than nearby rural areas. The annual mean air temperature of a city with one million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.”

Researchers from Yale University's School of Forestry and Environmental Studies found that the urban heat island effect was most pronounced in the warm and wet parts of the country. According to the study, it's because the smooth surfaces of the city don't release heat into the atmosphere, through a process known as convection, as efficiently as the rough surfaces of the forests and trees.

“In wet climates (such as warm damp southeastern US), we have lush forests,” Lei Zhao, a doctoral student and lead author of the study, told me. “Forests are efficient 'heat convertors' that dissipate heat away from the ground to the lower atmosphere. In comparison, cities in these climates are not as efficient in convecting heat—our estimate is that they are 58 percent less efficient than the natural landscape—and hence we have a strong urban heat island.”

For the rest of the story:

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