Understanding the Urban Heat Island Index

Urban Heat Islands

Large urbanized areas can 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 an 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).

According to the U.S. Environmental Protection Agency, daytime temperatures in urban areas are on average 1-6° F higher than in rural areas, while nighttime temperatures can be as much as 22° F higher as the heat is gradually released from buildings and pavement.

Why Heat Islands Matter

The concentration of heat in urban areas creates health risks both because of heat exposure and because of the enhanced formation of air pollutants, especially ozone. The strong influence of the urban heat island on nighttime temperatures limits the ability of people to cool down and recover before the heat of the next day, and therefore adds to the risk of illness and fatalities. Those most susceptible to heat include pregnant women, young children, the elderly, and people with certain preexisting conditions such as diabetes or heart disease, and people who work or exercise outdoors. These issues are summarized in the report Preparing California for Extreme Heat, published by the California Climate Action Team’s Public Health Workgroup.

Major increases in deaths, hospitalizations, and emergency room visits have been documented to occur during heat waves. Further, even during a non-heat-wave period, there are clearly documented associations between increased temperatures and a range of health problems. Increased hospital visits and emergency room visits have been documented in California from increased heat, including visits due to respiratory disease, emphysema, heart disease, heart attacks, stroke, diabetes, renal failure, intestinal infections, heat stroke, dehydration, hypertension, and asthma. For every increase of temperature by 10° Fahrenheit, there is a nearly 9 percent increase in preterm births. Many of these studies are summarized in the Human Health Impacts of Climate Change, a summary compiled by the Office of Environmental Health Hazard Assessment.

Although it would seem that hotter areas would have more intense urban heat islands and worse health effects, research has shown that surprisingly, people living in normally cooler areas may be more susceptible to health effects from temperature increases. For example, during the 2006 California heat wave, the greatest increase in emergency room visits occurred in the coastal cities even though the most intense heat was inland. This phenomenon may occur because fewer buildings are air conditioned in coastal areas, and people are less physiologically acclimated to heat.

Because urban heat islands result in locally higher temperatures, they also have significant effects on energy consumption. Additional air conditioning is required to counter-balance the increased temperatures, thereby increasing greenhouse gas emissions. This problem is especially serious because heat intensity is projected to increase significantly with climate change, thereby exacerbating the urban heat island cycle.

Urban Heat Island Index and Its Uses

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.¹ To address that gap, in 2012 the California Legislature required that the California Environmental Protection Agency (CalEPA) develop an Urban Heat Island Index (AB 296, Chapter 667, Statutes of 2012). The Index is designed 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 (PDF).” It defines and examines the characteristics of the urban heat island, and scientifically assigns a score based on atmospheric modeling for each census tract in and around most urban areas throughout the state. The study also produced Urban Heat Island Interactive Maps. The scores are based on hour-by-hour atmospheric modeling over two three-month-long summer seasons. The years modeled include 2006 (the year of a major heat wave) and 2013 (the most recent year for which data were available). The study was performed with guidance and peer review from a multi-agency scientific Project Oversight Workgroup, and was tested against actual meteorological data.

The Index may have a variety of uses. Urban heat islands may be priority areas for education and public health preparedness for extreme heat. Along with other environmental studies, such as CalEnviroScreen, the statewide map may help identify and prioritize areas across the state for mitigation or other actions. Governments may be interested in the maps of individual cities in the report 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 see changes in the Index in response to both climate change and mitigation efforts.

How to Interpret the Urban Heat Island Index

The Index is calculated as a positive temperature differential over time between an urban census tract and nearby upwind rural reference points at a height of two meters above ground level, where people experience heat. The Index is reported in degree-hours per day on a Celsius scale. An increase of one degree over an eight hour period would equal eight degree-hours, as would an increase of two degrees over a four-hour period. The degree-hour therefore combines both the intensity of the heat and the duration of the heat into a single numerical measure.

To perform an approximate conversion to a total number of degrees Fahrenheit per day, divide the Index by 24 hours and multiply the result by 1.8 degrees. For example, if the index is 120 degree-hours per day, then the approximate average temperature difference between rural and urban in that area is 9° F. Areas with greater temperature differentials over longer periods as compared to surrounding non-urban areas receive a higher Index score, demonstrating where the heat islands manifest most intensely across the state.

Some parts of the state have significantly higher temperatures, but these areas don’t necessarily have the most intense urban heat islands. That is because the more rural areas upwind of the urban area are also quite hot, so the differential between rural and urban may not be large. Therefore these maps do not reflect the total heat in each area, just the increase in temperature that is due to the urban heat island itself. For example, the average summer temperature in Fresno is about 84° F, and the urban heat island effect averages 4° F, which is a relatively modest increase over a hot background. In contrast, the average summer temperature in Ontario is lower at about 78° F, while the heat island effect is greater, averaging 9° F over background.

What the Study Found

The study showed that the Index tends to increase during heat waves, so that urban areas are hit harder than the surrounding areas. This effect is expected to become even more important in the future. With climate change, heat waves are becoming more frequent, more intense, and longer lasting. For many California cities, extreme heat days – daily high temperatures that used to occur about four times a summer – will occur 40 to 70 days during the summer by 2050, according to an analysis based on Cal-Adapt.

Another finding is the degree to which the urban heat island effect can be shifted by wind and topography. California’s climate is somewhat unique in that cool ocean water offshore contributes to cooling in coastal cities, while inland mountains trap warm air. As a result, the heat generated by urban heat islands in one area tends to move inland to blanket other areas with the overheated air. This phenomenon is clearly demonstrated in the San Francisco Bay Area, the Los Angeles Basin, and San Diego. An implication of this phenomenon is that mitigation may need to occur in areas upwind of those that are suffering the brunt of the impacts. A similar phenomenon occurs with ozone air pollution, which blows inland to disproportionately affect some of the same communities. In the Los Angeles basin, the geographic area most impacted by urban heat island effects has more than an 80 percent overlap with the area most affected by ozone pollution. This has important implications because both exposures can increase risk of some of the same cardiovascular and respiratory health effects.

Due to the wind and topography effects, the statewide version of the Index map shows fairly few distinctions between adjacent census tracts, and the association between local land use and the heat island effects appear less than anticipated. Rather, the effects are related to the size of the urban area. Small urban areas have average daily summer temperature increases up to 5° F, larger cities up to 9° F. For really large urban areas such as in Southern California, the urban heat islands blur together to form an urban heat archipelago, with average temperature increases up to 19° F at the eastern end of the basin.

The area of Riverside-San Bernardino was found to suffer from a surprisingly intense urban heat island, which dominates and affects the overall scale of the statewide map. It can be difficult to visually discern small differences on this scale, so it is helpful to click on individual census tracts to see the actual calculated score for each tract. Alternatively, Appendix C (PDF) of the report contains individual maps for each modeled urban area. These maps are each on a different color scale, reflecting a more optimal way to visualize differences within each city, as opposed to between cities. Just because an area shows as blue or green on the statewide map does not mean that the urban heat island effect is not significant for that area. Blue and green represent significant temperature increases above background, in the range of 4° F, and these urban heat islands may also be considered for mitigation.

¹ The closest tool that exists is U.S. EPA’s Mitigation
Impact Screening Tool (MIST), which provides qualitative
assessments of the likely impacts of heat island mitigation
strategies averaged at the city-scale.


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