Urban Heat Island Index for California

Large urban areas often experience higher temperatures, greater pollution, and more negative health impacts during hot summer months, when compared to more rural communities. This phenomenon is known as the urban heat island. Heat islands are created by a combination of heat-absorptive surfaces (such as dark pavement and roofing), heat-generating activities (such as engines and generators), and the absence of vegetation (which provides evaporative cooling).

Creating the Index

Until now, there has been no way to quantify the extent and severity of an urban heat island for individual cities. There was no consistent “index” to define an urban heat island, and no maps to show where and how intensely they manifest at a local scale. To address that gap, in 2012 the California Legislature required that the California Environmental Protection Agency develop an Urban Heat Island Index (AB 296, Chapter 667, Statutes of 2012) and to design it so that “cities can have a quantifiable goal for heat reduction.”

In 2015, CalEPA released a study entitled, “Creating and Mapping an Urban Heat Island Index for California (5 MB, PDF). ” It defines and examines the characteristics of the urban heat island and, for the first time, creates an Urban Heat Island Index to quantify the extent and severity of urban heat islands for individual cities. The study also produced Urban Heat Island Interactive Maps, showing the urban heat island effect for each census tract in and around most urban areas throughout the state.

Uses of the Index

The index may have a variety of uses. Urban heat islands may be priority areas for public health education and preparedness for extreme heat. Along with other environmental studies, such as CalEnviroScreen, this research can also help identify and prioritize areas across the state for mitigation. Governments may be interested in the maps of individual cities as a tool for prioritizing local activities such as urban greening and projects focused on cooler roofs and pavements. Over time, it may be possible to track changes in the index that result from climate change or mitigation efforts.