On a warm summer day, the air in a city can be 6-8 degrees Fahrenheit hotter than surrounding areas that have large tree canopies. Trees intercept solar radiation and transpire water. When the effects of this evapo-transpiration are combined with the effect of strategically placed shade trees, temperatures can drop by as much as 9 degrees Fahrenheit. Cities without adequate tree cover create "urban heat islands."

City of Atlanta Heat Island Effect

Credit: NASA/Goddard Space Flight Center
Scientific Visualization Studio

Visit this page on the Environmental Protection Agency's website for comprehensive information about heat islands, their impacts, and how to mitigate them.