Los Angeles Skyline
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Feeling the Heat in US 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 5°F 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 over 60% of the world’s population living in cities and extreme heat becoming more frequent over the past 20 years across the contiguous 48 states, cities are at an increased vulnerability to heat waves and a changing climate.

Learn More

  • The image below shows how air and surface temperatures vary over different land use areas.

urban heat island
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  • The image below shows the percentage of the land area of the contiguous 48 states with unusually hot daily high and low temperatures during the months of June, July, and August, 1910-2015. 

summer temperatures
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What You Can Do

Sources

  • 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., 52, 2051–2064, https://doi.org/10.1175/JAMC-D-13-02.1  
  • Environmental Protection Agency. 2017. "Climate Change and Heat Islands." Accessed online July 18, 2017. https://www.epa.gov/heat-islands/climate-change-and-heat-islands
  • Environmental Protection Agency. 2017. “Heath Island Effect”. Accessed online July 18, 2017. http://www.epa.gov/heatisland/index.htm
  • United States Census. 2015. U.S. Cities are Home to 62.7 Percent of the U.S. Population, but Comprise Just 3.5 Percent of Land Area. Accessed online July 18, 2017. https://www.census.gov/newsroom/press-releases/2015/cb15-33.html 
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