Climate Fact: Feeling the Heat in Our Cities?

Do you live in an urban area? If so, you may be living on an “island” – not a tropical island, but an urban heat island.

A city of one million people or more can be up to five degrees Fahrenheit hotter than its rural counterparts, and the temperature difference can be even more pronounced at night. Urban heat islands form in cities for several reasons: they have higher amounts of asphalt and concrete that absorb and slowly release heat, a lower ability to reflect the Sun’s radiation, lack surface moisture and fewer green spaces. Heat waves – products of high pressure systems that increase both air and surface temperatures – can increase the amount of moisture returned to the air, slow wind speeds and strengthen secondary air circulations. Heat waves interact with urban heat islands to produce an effect greater than the sum of the background urban heat island effect and heat wave effect. This intensifies the difference between urban and rural temperatures and creates more heat stress in cities. With 50 percent of the world’s population living in cities and heat waves doubling in frequency over the 20th century, cities are at an increased vulnerability to heat waves and climate change.

Learn more about protecting your health during a heat wave.

Image: Air and surface temperatures vary over different land use areas. Click to download. Courtesy of U.S. EPA.





















(Source: Li, D., and E. Bou-Zeid, 2013: Synergistic Interactions between Urban Heat Islands and Heat Waves: the Impact in Cities is Larger than the Sum of its Parts. J. Appl. Meteor. Climatol. doi:10.1175/JAMC-D-13-02.1, in press. and Environmental Protection Agency. “Heath Island Effect”. Online Posting, 06 March 2006. Accessed online 10 July 2013 <>)

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