First-of-Its-Kind Index Quantifies Urban Heat Islands

For Immediate Release

September 16, 2015
Contact: Alex Barnum, (916) 324-9670


SACRAMENTO – The California Environmental Protection Agency released a study today that identifies areas across California experiencing hotter summer temperatures as a byproduct of urban development.

Urban areas have higher temperatures during summer compared with adjacent rural communities, a phenomenon known as the urban heat island. Heat islands are created by a combination of dark pavement and roofs that absorb heat, heat-generating activities such as engines and generators, and the absence of vegetation that provides evaporative cooling.

While the phenomenon is well-known, the study for the first time creates an Urban Heat Island Index to quantify the extent and severity of the heat island effect for individual cities throughout the state. The study found temperature increases ranging from a few degrees in small cities and coastal areas to as much as 19 degrees on average over a day in large, inland urban areas.

“Until now there was no way to measure the urban heat island effect in California,” said California Secretary for Environmental Protection Matthew Rodriquez. “The Urban Heat Island Index offers state and local governments a tool to help prioritize areas for programs that reduce heat in urban communities, such as urban greening projects and cooler roofs and pavements.”

Concentration of heat in urban areas is a concern because it creates health risks from heat exposure and the increased formation of air pollutants, particularly ground-level ozone or smog. It also affects energy consumption through additional air conditioning needed to counter-balance the higher temperatures. The urban heat island effect is projected to increase with climate change.

Last year was the warmest year on record since1880, when modern temperature records were first kept, according to NASA and the National Oceanic and Atmospheric Administration. All 10 of the hottest years have come since 1998, and this year is on track to break another record.

“Because of climate change, we can expect to see heat waves becoming more frequent, more intense and longer lasting,” said Dr. Gina Solomon, CalEPA’s Deputy Secretary for Science and Health. “Knowing an area’s Index score could help communities target outreach and education to protect against heat illness in vulnerable populations such as children and the elderly.”

The index assigns a score for each census tract in and around most urban areas throughout the state. The scores are based on atmospheric modeling over two three-month-long summer seasons – 2006 (the year of a major heat wave) and 2013 (the most recent year for which data were available). Interactive maps showing these scores are available at local or statewide scale.

Among the study’s findings:

  • Hotter parts of California don’t necessarily have the most intense urban heat islands. For example, the heat island effect in Fresno averages 4° F, a relatively modest increase over the already high average summer temperature of about 84° F. In contrast, the average summer temperature in Ontario is about 78° F, while the heat island effect is greater, averaging around 9° F.
  • Wind and topography can shift the urban heat island effect. In major coastal cities, cool ocean air blows heated urban air inland where it gets trapped against mountain ranges. As a result urban heat generated in one area tends to move inland to blanket other areas with the overheated air. A similar phenomenon occurs with ozone air pollution.
  • Heat island effects are related to the size of the urban area. Large urban areas have average daily summer temperature increases up to 9° F compared with non-urban regions, while smaller cities average an increase of up to 5° F. The largest effect is in southern California, where the urban heat islands blur together to form an “urban heat archipelago” with average temperatures up to 19° F higher in the Riverside-San Bernardino region at the eastern end of the basin.

The study, “Creating and Mapping an Urban Heat Island Index for California,” was conducted in response to legislation (AB 296, Chapter 667, Statutes of 2012) that directed CalEPA to develop a heat island index so that “cities can have a quantifiable goal for heat reduction.” The study was performed with guidance and peer review from a multi-agency Project Oversight Workgroup.

The study, maps and a fact sheet are available on the Urban Heat Island Index webpage.

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Air Resources Board • Department of Pesticide Regulation • Department of Resources Recycling and Recovery • Department of Toxic Substances Control • Office of Environmental Health Hazard Assessment • State Water Resources Control Board • Regional Water Quality Control Boards

1001 I Street, Sacramento, CA 95814 • P.O. Box 2815, Sacramento, CA 95812 • (916) 323-2514 www.calepa.ca.gov