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Heat Islands

Heat Island Newsroom

Two men and one woman in a park with trees read from a clipboardWelcome to EPA's Heat Island Newsroom, which includes recent issues of the Heat Island Newsletter and other notable news items. Older newsletters can be found in the Newsroom Archive. To receive the newsletter and notices of heat island-related conferences by email, sign up for EPA's Heat Island Newsletter.

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April 4, 2019 Newsletter

National Public Health Week LogoIt’s National Public Health Week! Did you know that heat islands can cause health problems? But heat island mitigation strategies can help lessen these problems. For example, installing green roofs can improve air quality, reduce heat stress, and make indoor environments cooler and more comfortable. Throughout National Public Health Week and this summer look for @EPA’s tweets about heat islands and other environmental health topics.

General Heat Islands

  • Identifying Heat Islands Large and Small – In a new study from Yale University, scientists developed a methodology to estimate surface urban heat island intensity. Researchers used satellite images to calculate heat island intensity for over 9,500 urban clusters over 15 years and used existing multi-city studies to validate their findings. Whereas most heat island studies focus on single cities or major cities, the new methodology is global in scale and does not discriminate by city size.
  • The Night and Day of Cooling Strategies – In a recent research article, Arizona State University scientists looked at the interaction between climate change, urban expansion, and the heat island effect. Unlike many studies, which use daily average temperatures, this study took into account days and nights separately. The scientists showed that while total implementation of cool roofs and street trees in cities can decrease projected average daytime heat, nighttime heat remains high. This is an important consideration for heat island reduction, because high nighttime temperatures can often be more dangerous for people’s health than high daytime temperatures.
  • Aerial imagery of Washington DC’s heat island in the morning, afternoon, and eveningData Available from Volunteer Heat Island Mapping Study – Researchers from Portland State University released the first results of volunteer heat island data collection efforts for Baltimore, MD, Washington, DC, and Richmond, VA. The study was unique in using both satellite and ground measurements from vehicle traverses and utilizing a citizen science approach. The results showed that the integration of these measurement methods accurately mapped each city's heat island, and that temperature differences corresponded to different levels of vegetative land cover.

Heat Waves

  • U.S. Heat Wave Frequency and Length are Increasing – Between 1961 and 2017, the heat wave season became longer among cities in the U.S. and heat waves occurred with greater frequency. Looking across 50 major cities, the U.S. Global Change Research Program’s scientific indicator is based on an EPA analysis and National Oceanic and Atmospheric Administration data. It depicts the upward trends in heat wave frequency and the length of heat wave season, and provides an overview of the associated human health impacts. Compounding these heat wave trends, the heat island effect is expected to strengthen as the structure, spatial extent, and population density of cities change and grow.

Cool Pavements

  • Cool Pavements are a Promising Strategy for Temperature Reduction – In a recent paper, scientists from the University of Salford, Arizona State University, and the University of Southern California asked the question, how well do different heat island mitigation strategies work? They used a high-resolution model of air temperature and movement to compare the effect of adding green roofs, cool roofs, additional trees, and cool pavements in the neighborhood of El Monte, CA. They found that cool pavements and additional trees each resulted in heat radiation reductions. Scaling up these strategies could lead to air temperature reductions.

Cool Roofs

  • Aerial view of flat white, gray, and green roofsMillion Cool Roofs Challenge Launched – In February, a group of organizations, including the Kigali Cooling Efficiency Program and Global Cool Cities Alliance, announced the Million Cool Roofs Challenge, a global grant program that aims to help developing countries employ cool roof technologies. Applications are open until May 20, 2019, and the winning team will receive $1 million to scale up their innovative cool roof design.

Equity

  • Equalizing Access to Heat Mitigation Strategies in Tucson – Tucson, AZ, is undertaking many climate and drought mitigation strategies, including promoting green infrastructure, and offering rebates and grants for residents to improve their neighborhoods by planting trees. However, in Tucson’s low-income neighborhoods, which are already hotter due to lower tree canopy coverage, these incentives are comparatively underused by residents. The article explains several reasons and actions Tucson is taking.
  • Why Refuse a Free Tree? Pitfalls of Policies to Increase Canopy Coverage – A partnership between the city of Detroit, MI, and the nonprofit The Greening of Detroit offered free trees, along with education and maintenance, but residents in low-income areas with fewer trees were not taking advantage of the opportunity. Researchers found several reasons why: people were accustomed to their neighborhoods being neglected by the city and thought the maintenance of the tree was likely to fall to them, and education was not well-targeted to the local lifestyle and values, among other factors. These findings could be valuable to other cities interested in implementing tree-planting programs.
  • Social Inequity and Urban Greenery – In January, scientists from the University of British Columbia produced a study that investigates the relationship between urban greenery and socioeconomic equity in 10 U.S. cities. Using high-resolution land cover data and census data, they found a strong positive correlation between vegetation and populations with higher income and education levels. They also found a negative correlation between vegetation and the proportion of racial and ethnic minorities.
  • Tree Canopy Policies Can Perpetuate Inequality – Researchers at the University of North Carolina, using geospatial data of Durham and Chapel Hill, NC, showed that lower tree canopy coverage was correlated with a higher percent of non-White population. The study was unique because the authors also assessed city policies and ordinances related to trees, in order to connect them with outcomes. They found that policies were likely to perpetuate existing gaps in citywide vegetation and tree coverage. The authors discussed ways policy makers can consider neighborhood disparities in green space policies.

Green Roofs

  • Hospital hallway with green plantsBenefits of Green Roofs for Hospitals – In its winter issue, Living Architecture Monitor noted that the environment within health care facilities can have an impact on patients and care providers, and that green spaces like green roofs can have a positive effect. In addition to reducing heat, green roofs provide a serene space, potentially contributing to reductions in patient medication levels, reduced length of hospital stays, and reduced patient stress. One example is the green roof atop Mercy Health-West Hospital in Cincinnati, OH, which features a therapy terrace for rehabilitation, storm water runoff capture, and a beautiful view for patients and staff.
  • Cities Drive Growth in Green Roof Market – From San Francisco, CA to Chicago, IL, more cities are turning to green roofs to reduce extreme heat. City policies like San Francisco’s Better Roofs Ordinance and Portland’s living roof initiative are driving economic growth in the roofing market while helping cities reduce the heat island effect.
  • City- and Regional-Scale Green and Cool Roof Deployment – In a modeling study conducted for six U.S. cities, researchers from Princeton University and The Hong Kong University of Science and Technology estimated the cooling efficiency of green roofs and cool roofs at city and regional scales. Should city centers deploy 25% green roofs and/or cool roofs they could cool up to 0.86°C, but results vary based on wind conditions and city structure. Researchers found regional benefits to green and cool roofs, but with lower cooling efficiency.

Smart Growth

Trees and Vegetation

  • The History of Street Trees – In a new book, Sonja Dümpelmann explores the history of street trees in New York, NY, and Berlin, Germany. Trees were not always part of the urban landscape. Dümpelmann chronicles their early supporters, who advocated for cooling and aesthetic benefits of trees, all the way to modern scientists, who recognize the ability of urban vegetation to counteract the heat island effect.

December 11, 2018 Newsletter

EPA Updates

  • Report Highlights Heat, Vulnerable Populations, and Public Health – EPA’s National Center for Environmental Assessment released a new report, Mapping the Vulnerability of Human Health to Extreme Heat in the United States. EPA and partners developed the report to support vulnerability assessments of state and local health departments, community planners, and other stakeholders as part of greater adaptation and resiliency endeavors. The report provides a foundation for how to determine and map vulnerable populations, taking sensitivity, adaptive capacity, and exposure to extreme heat into consideration.

General Heat Islands

  • Heat Islands are a Cross-Cutting Issue in the Fourth National Climate Assessment – Volume II of the Fourth National Climate Assessment, released in November 2018, details impacts, risks, and adaptation to climate change in the United States. Heat islands cut across topics and sectors, appearing in chapters on land-use change, the built environment, and human health. The authors project that continued climate change will exacerbate heat islands in urban areas. In addition, the assessment links warming temperatures to a range of illnesses and deaths, particularly for vulnerable populations including older adults, pregnant women, and children.
  • The Twin Cities Map a Path to Reduce Local Heat Island Effects –As summer heat waves bring increasingly high temperatures to Minneapolis and St. Paul, Minnesota, the Twin Cities Metropolitan Council released its Climate Vulnerability Assessment. The assessment includes a number of tools such as detailed maps on extreme heat vulnerability in the region and a story map about heat. The Keeping Our Cool Story Map provides an interactive forum to educate the public about weather and climate, the heat island effect, and related mitigation activities in the Twin Cities.
  • ECOSTRESS image showing heat patters in Los AngelesImage: NASA/JPL-CaltechECOSTRESS Imagery Captures Surface Temperature Patterns – Gliding through space on the International Space Station, the National Aeronautics and Space Administration’s (NASA’s) new ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) measures the Earth’s surface temperatures. ECOSTRESS can help scientists identify heat islands and study how different surfaces heat up and cool down. The instrument is already being used to measure surface temperature variations in Los Angeles, where it can detect heat distribution
    over areas the size of a football field and collects data multiple times per day.
  • Portland Researchers Show Temperature Varies across City – Researchers at Portland State University (PSU) are using special thermometers to record and map temperatures throughout the City of Portland, Oregon. In addition to mapping heat islands, PSU researchers are also working to reduce them. The team is engaging with the Portland Bureau of Planning and Sustainability to incorporate design principles into city construction codes that help mitigate heat, such as increasing air circulation between buildings.
  • Areas of Washington, DC and Baltimore, Maryland Heat Up – In summer 2018, volunteers recorded temperatures in Washington, DC and Baltimore as part of the field research led by the National Oceanic and Atmospheric Administration. In October, project partners released a series of maps profiling the results of the research. In Baltimore, the hottest areas recorded were central neighborhoods with high concentrations of asphalt and concrete surfaces. In Washington, DC, on August 28, 2018, the temperatures recorded varied across neighborhoods by 17 degrees. In both cities, many cool spots on the maps overlapped with parks.

Green Roofs

  • Green Roofs Reduce Heat Stress in Vulnerable Communities – Researchers from the University of Notre Dame developed an approach to identify neighborhoods where the installation of green roofs could most effectively mitigate heat while providing co-benefits such as improved health outcomes. The study integrated a variety of information about social vulnerability, temperature simulations, and electricity consumption in Chicago. The methods could be adopted by other urban regions seeking to investigate where green roofs might have the greatest impact.
  • Front view of ACROS Fukuoka Prefectural International Hall in Fukuoka, Kyushu, Japan.Green Roofs across the World – Green roofs are found all over the world in all shapes and sizes! Take a look at some large-scale green roofs and rooftop gardens in this photo gallery.

Cool Roofs and Walls

  • Denver Opts for Cool Roof Solution – In October 2018, the Denver City Council voted to modify the green roof requirement that city voters had approved as a ballot initiative last year. Instead of green roofs, the new law will require cool roofs, which have reflective surfaces designed to minimize heat absorption, on new and re-roofed buildings over 25,000 square feet. In addition to cool roof installations, these properties must choose one of several “green” options: installing a green roof/green space, funding it elsewhere, meeting green building certification standards, installing renewable energy, or undertaking energy efficiency.
  • Technical Assessment of Cool Walls – Researchers from the University of Southern California and Lawrence Berkeley National Laboratory recently published an assessment of cool walls in Los Angeles. The team used a regional climate model coupled with an urban canopy model to investigate how the addition of cool walls could influence solar reflection and surface air temperatures. The authors found temperature benefits of cool walls even after accounting for reflected solar radiation that was partially absorbed by surrounding walls and pavements.

Smart Surfaces

  • Smart Surfaces Coalition Forms to Reduce Urban Heat – A group of organizations, including the Global Cool Cities Alliance, the Urban Sustainability Directors Network, the National League of Cities, Smart Growth America, and the U.S. Green Building Council, has come together to form the Smart Surfaces Coalition, which aims to help municipalities adopt technologies that reduce heat and save money. Smart surfaces like cool roofs and green roofs deliver a range of environmental and social benefits relative to conventional surfaces.

Trees and Vegetation

  • Cooling the Dallas Concrete Jungle – The Trust for Public Land, working with the Texas Trees Foundation and The Nature Conservancy, overlaid tree coverage maps with socioeconomic and health data on Dallas neighborhoods to determine high-priority areas for tree planting. With this information, volunteers have a goal of planting 1,000 trees in the Oak Cliff neighborhood, an area where high poverty rates and low canopy coverage increase residents’ vulnerability to the heat island effect.

Public Health

  • Woman drinking from reusable water bottleClimate Change, Health, and Equity Guide – The Public Health Institute and the American Public Health Association recently published a comprehensive guide for local health departments on climate change and health. The Extreme Heat chapter provides a background on heat islands, the impacts of extreme heat on health and health equity, and what local health departments can do to address vulnerability to heat.
  • What are the Health Impacts of Heat? Watch this Video! – The Centers for Disease Control and Prevention released a short and easy-to-share public service announcement about preparing for extreme heat.

September 6, 2018 Newsletter

General Heat Islands

  • Reduced Heat Island Intensity under Warmer Conditions – Researchers from John Hopkins University recently published findings that the intensity of the heat island effect decreases as temperature increases. They studied 54 locations in the United States from 2000 to 2015, looking at the daily minimum and maximum temperature differences between paired urban and rural weather stations. The difference in rural and urban temperatures decreased with temperature increases in 70% of the locations. The researchers find that the results are driven by changes in rural temperatures rather than urban temperatures.
  • Baltimore, Maryland cityscapeVolunteers Map Heat Islands in Washington, DC, and Baltimore, MD – A project led by the National Oceanic and Atmospheric Administration is using volunteers in vehicles equipped with special thermometers to determine which areas of Washington, DC, and Baltimore, MD, are the hottest. This information could identify which communities need assistance during heat events, or where investments are needed for cooling strategies, such as cool roofs or green roofs.
  • From Los Angeles, CA, to Louisville, KY, U.S. Cities Seek Heat Island Solutions – As warming trends become more pronounced, large cities must find solutions to resurface the urban environment, slow the rate of warming, and protect public health. Researchers encourage cities to think beyond a single solution, and pursue a palette of strategies including the use of shade trees, walkable corridors, and altering land and energy use decisions.
  • The High Cost of Hot – Cooling degree days (CDDs) are used to estimate the amount of artificial cooling needed to maintain a comfortable indoor temperature. A new report by Climate Central, an independent organization that focuses on climate research, profiles how an increase in daily minimum temperatures, and CDDs in particular, are associated with higher energy bills from the use of air conditioning. The report explores changes in 211 cities in the United States; 93% have seen an increase in CDDs since 1970, including cities with more moderate climates. Much of this increase is attributed to increased low temperatures at night.
  • New Video Succinctly Describes the Heat Island Effect – The online video produced by National Public Radio gives a quick overview of the heat island effect that is suitable for a majority of audiences.

Heat Waves

  • Keeping Cities Cooler during Heat Waves – A recent news article in The New York Times profiles several strategies for cities to prepare for and cope with heat waves. It points to some lesser-known strategies including the creation of ventilation corridors that restrict the construction of buildings in certain locations, permitting unrestricted air to flow and bringing down nighttime temperatures. Preparing for blackouts can also help cities cope with heat waves and protect public health.
  • Buildings Feel the Heat, Too – Researchers from Arizona State University looked into how buildings constructed to comply with building energy codes fared in a hypothetical three-day power outage. They found that in most climates, indoor conditions became too hot to be considered safe for occupants. However, they observed a link between building energy efficiency and resiliency to heat – more energy efficient buildings fared better in warmer climates.

Green Roofs

  • Portland Integrates Green Roofs as Part of City Plan – Portland, OR, recently adopted the Portland Central City 2035 Plan with a mandate for vegetated roofing on buildings larger than 20,000 square feet. A cohort of partners evaluated this component of the new plan; they include the Green Roof info Think-tank (GRiT), Green Roofs for Healthy Cities, Greenroofs.com, the Audubon Society, and Portland State University.
  • Sprinkler spraying purple and red flowersIrrigation Improves the Cooling Benefit of Green Roofs – In a research article featured in Building and Environment journal, researchers modeled the effectiveness of green roofs under different irrigation scenarios. Green roofs with moderate irrigation reduced excess heat by 15-51% compared to conventional roofs, and green roofs with unrestricted irrigation reduced excess heat by 48-75% compared to conventional roofs.

Green Infrastructure

  • New Green Infrastructure and Health Guide – A new guide produced by the Willamette Partnership, the Oregon Public Health Institute, and the Green Infrastructure Leadership Exchange explores the linkages between green infrastructure and public health. The guide provides best practices about how to use green infrastructure to promote health equity including identifying community health needs, making the business case for health and green infrastructure, engaging the public, siting and designing projects, and evaluating health benefits.
  • Literature Review of the Cooling Effects of Green Infrastructure – A literature review published in Solar Energy takes a compressive look at how cooling effects of green infrastructure are documented in published literature. The authors evaluated 165 studies from 2010-2017 on a range of criteria including geographic focus, benefits analyzed, and type of green infrastructure. They found a number of gaps in the literature including oversight of the cumulative effects of green infrastructure. They also noted the green infrastructure field could benefit from standard protocols or classification systems.

Cool Roofs

  • Effect of Cool Roofs at a Neighborhood Scale – Researchers from Lawrence Berkeley National Laboratory published the results of a theoretical analysis, modeling the potential impact of a neighborhood-scale cool roof demonstration project on air temperature. Their results show a potential reduction in air temperature ranging from 0.5°C to 1.3°C, depending on the heat transfer analysis method used. They caution more modeling is needed before beginning a neighborhood-scale demonstration project.

Trees and Vegetation

  • U.S. Urban Tree Cover Declining – Researchers from the U.S. Forest Service report that the United States might have lost as many as 36 million trees per year from 2009 to 2014. The researchers used aerial photography to compare land cover changes in urban areas across the entire United States. They found that tree cover is declining at a rate of 175,000 acres per year. Jurisdictions with the greatest decline in tree cover, in descending order, were Oklahoma, the District of Columbia, Rhode Island, Oregon, and Georgia, coinciding with a gain in impervious surface cover. They note the range of benefits that urban forests provide, and consequently recommend integrated programs that focus on sustaining tree canopy.

Public Health

  • Washington Square Park in NYC with archway and large fountainCool Neighborhoods in New York City – In New York City, officials in the Office of Recovery and Resiliency, Health Department, and partner organizations are working together to protect public health during extreme heat events. The $106 million Cool Neighborhoods program includes painting surfaces white, planting trees, constructing green roofs, and building green infrastructure to cool specific neighborhoods. The program targets heat-vulnerable communities, such as those that lack vegetation, or have at-risk populations, such as older adults.

May 30, 2018 Newsletter

General Heat Islands

  • Bird's eye view of a gridlike city with tree-lined streetsHeat Islands and the Patterns of Cities – A study from the Massachusetts Institute of Technology found that grid-like cities had a greater buildup of heat than cities with less-orderly patterns, and that the arrangement of buildings and streets was the most important determinant of the heat island effect. According to the authors, the layout influences the way buildings absorb and re-radiate heat to surrounding structures, which may account for the temperature differences observed.
  • Mitigating the Heat Island in Gowanus, Brooklyn – Gowanus is an area of South Brooklyn, NY, with a high proportion of low-income residents and a lack of parks and greenery, factors that increase its population’s vulnerability to the heat island effect. The Urban Land Institute’s recent report proposes comprehensive heat mitigation strategies that could cool the neighborhood locally and improve health outcomes (e.g., a new park, increased tree canopy cover, and efficient building design).
  • A Zonal Model for Street Canyon Air Temperature – The buildout of cities with tall structures lining narrow streets creates street canyons, or urban canyons, that can exacerbate the heat island effect. Researchers from the University of Hong Kong and contributors recently published a new model for measuring canyon air temperatures. By incorporating three-dimensional landscapes, wind movement, and heat transfers between surfaces, the model reliably predicted street canyon air temperatures in zones with different street layouts or building height-to-width ratios. The zonal model could be used to assess the impact of these canyons on heat islands in high-density cities.

Heat Waves

  • How Do Heat Islands Interact with Heat Waves? – The heat island effect increases cities’ baseline temperature, which can exacerbate the effects of heat waves. A team of researchers led by Princeton University modeled this interaction for 50 U.S. cities. They found that the synergistic effects between heat islands and heat waves are currently most significant in eastern and southeastern cities, but projected that in the future, the effects will be more pronounced for cities in arid climates in the Southwest.
  • State and Local Resilience to Extreme Heat – The Center for Climate and Energy Solutions developed a fact sheet to help state and local officials consider emergency responses to heat waves as part of a holistic approach to managing climate risks. The fact sheet includes costs, benefits, and co-benefits of different resilience strategies like using cool pavements and increasing tree canopy cover. It also provides real-world examples from cities that have implemented these approaches.
  • Cityscape glowing in orange under the sunStudy Researches Main Cause of Future Heat Waves in United States – According to scientists from the University of Miami and the National Oceanic and Atmospheric Administration, rising temperatures will surpass natural variability as the main cause of future heat waves in some parts of the United States. The authors of the study determined this will occur in the western United States by the late 2020s and in the Great Lakes region by the mid-2030s.

Cool Roofs

  • Cool Roof Rating Council’s (CRRC’s) New Online Rating Portal for Roofing Products – CRRC evaluates the solar reflectance and thermal emittance of roofing products, which helps roofing manufacturers and purchasers meet heat island reduction and building energy-efficiency goals. In April 2018, the CRRC Product Rating Program launched its new online rating portal. This new portal streamlines the rating application process and improves CRRC’s product directory. It helps manufacturers better manage their applications for obtaining product ratings, including ENERGY STAR® certification.
  • Landscape with an urban center surrounded by rural areas and farmlandReflective Surfaces Help Urban Heat Islands – and Rural Ones Too – Yale Environment 360 reported on some of the latest cool roof, green roof, and reflective surface research and technologies, which have the ability to reduce the heat island effect in urban and rural areas.

Green Infrastructure

  • Delivering Urban Resilience: Costs and Benefits of Adopting Smart Surfaces – A report written by Capital E and recently released by the U.S. Green Building Council analyzed the costs and benefits of “smart” surfaces, or surfaces that deliver a range of environmental, economic, health, and social benefits relative to conventional surfaces, across three U.S. cities: El Paso, TX; Philadelphia, PA; and Washington, DC. The authors demonstrated that smart surfaces, including green roofs, cool roofs, solar panels, and permeable and reflective pavements, offset extreme heat and extreme weather driven by warming; while also mitigating climate change, stormwater runoff, and netting billions of dollars in financial benefits.

Trees and Vegetation

  • Urban Trees and Water Availability – Researchers from Harvard and North Carolina State University previously found that the heat island effect was associated with increased pests and reduced growth of the willow oak, a common large shade tree in the southeastern United States. In a recent study, the team linked reduced tree growth to water stress, and recommended that urban forest management strategies focus on tree hydration to mitigate the effects of warming and drought.
  • Why Dallas is Warming Up and Plans to Cool Down – The 2017 Dallas Urban Heat Island Management Study from the Texas Trees Foundation, in collaboration with the Georgia Institute of Technology, found that more than one-third of Dallas is covered by impervious surfaces like concrete and commercial buildings, which absorb heat and contribute to the heat island effect. The foundation’s report provides a framework for maintaining existing trees and planting thousands of new ones to increase the city’s urban forest canopy, thereby cooling the city and cleaning the air.