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NASA

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An urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas.

Why does this happen?

An urban area is a city. A rural area is out in the country. The sun's heat and light reach the city and the country in the same way. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat.

If you travel to a rural area, you'll probably find that most of the region is covered with plants. Grass, trees and farmland covered with crops, as far as the eye can see.

Plants take up water from the ground through their roots. Then, they store the water in their stems and leaves. The water eventually travels to small holes on the underside of leaves. There, the liquid water turns into water vapor and is released into the air. This process is called transpiration. It acts as nature's air conditioner.

You can feel cooling transpiration at work on a hot summer day. On a sunny day, go outside and find a sidewalk that is right next to a patch of grass. Feel both surfaces. The grass should feel cooler on your skin than the pavement-and that's mostly due to transpiration!

When you visit a big city, you won't see many plants. Instead, you'll see sidewalks, streets, parking lots and tall buildings. These structures are usually made up of materials such as cement, asphalt, brick, glass, steel and dark roofs.

What do urban building materials have in common?

First of all, materials such as asphalt, steel, and brick are often very dark colors-like black, brown and grey. A dark object absorbs all wavelengths of light energy and converts them into heat, so the object gets warm. In contrast, a white object reflects all wavelengths of light. The light is not converted into heat and the temperature of the white object does not increase noticeably. Thus, dark objects-such as building materials-absorb heat from the sun.

To cool down urban heat islands, some cities are 'lightening' streets. This is done by covering black asphalt streets, parking lots, and dark roofs with a more reflective gray coating. These changes can drop urban air temperatures dramatically, especially during the heat of summer.

Planting gardens on urban rooftops can also help to cool down the city, too! In fact, a study in Los Angeles, California, calculated that changes like these would be enough to save close to $100 million per year in energy costs!

Urban building materials are another reason that urban areas trap heat. Many modern building materials are impervious surfaces. This means that water can't flow through surfaces like a brick or a patch of cement like it would through a plant. Without a cycle of flowing and evaporating water, these surfaces have nothing to cool them down.

To help cool the heat island, builders can use materials that will allow water to flow through. These building materials-called permeable materials-promote the capture and flow of water, which cools urban regions.

What does it mean?

Urban heat islands are one of the easiest ways to see how human impact can change our planet. After all, sidewalks, parking lots and skyscrapers wouldn't exist if humans weren't there to build them. And although these structures are essential to city living, the heat islands they create can be dangerous for humans.

In the summer, New York City is about 7°F (4°C) hotter than its surrounding areas. That doesn't seem like much, but these higher temperatures can cause people to become dehydrated or suffer from heat exhaustion. The hot temps also require more energy to operate fans and air conditioners. This can lead to power outages and a serious danger to public health.

But, there are things we can do to help cool the cities down. And NASA satellites can help to figure out where these cities are the hottest.

Earth-observing satellites, such as Landsat and Suomi-NPP, can keep a close eye on the Earth's vegetation and surface temperature. Scientists can use this information to track hotspots in cities across the planet. NASA scientists, with their global satellite views, are working to understand urban heat islands and help urban planners to build more energy efficient, cooler and safer cities.

Related NASA Missions

Related Resources for Educators

Story Map: Creation of Urban Heat Islands
Urban Heat Island Lessons and Activities

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An urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas. An urban area is a city. A rural area is out in the country. The sun's heat and light reach the city and the country in the same way.

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Climate Change Resilience: Cooling an Urban Heat Island

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Climate Change Resilience: Cooling an Urban Heat Island

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American Museum of Natural History | 2018

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Climate Change Resilience: Cooling an Urban Heat Island

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NPR

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Low-income areas of cities are often hotter than their wealthier counterparts, and that heat can have dire health consequences.

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US EPA

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EPA's Heat Island Effect Site provides information on heat islands, their impacts, mitigation strategies, related research, a directory of heat island reduction initiatives in U.S. communities, and EPA's Heat Island Reduction Program.

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The Weather Channel | 2013

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Extreme Summer Heat & Urban Heat Island Effect Explained

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Urban Heat Islands Causing Global Warming

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The Weather Channel meteorologist Mark Elliot explains why cities are usually warmer than the suburbs.

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Do hot neighborhoods affect everyone equally? (Audio Version) 

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Do hot neighborhoods affect everyone equally? (Audio Version) 

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[Audio version] Do hot neighborhoods affect everyone equally?

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What can we do about urban heat? What's already being done? 

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What can we do about urban heat? What's already being done? 

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NIHHIS helps decision makers prepare for extreme heat events days, months, and years in the future.

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NPR | 2018

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Why It’s Usually Hotter In A City | Let's Talk | NPR

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How would design a city of the future to mitigate the urban heat island effect? 

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How would design a city of the future to mitigate the urban heat island effect? 

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"Urban heat islands" occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. This effect increases energy costs (e.g., for air conditioning), air pollution levels, and heat-related illness and mortality.

Climate change will likely lead to more frequent, more severe, and longer heat waves during summer months. See: 100-degree-days figure (JPG).

The city of Chicago could see 30 more days per year rise above 100 degrees Fahrenheit (°F) under "high" greenhouse gas emissions scenarios.

Under lower emissions, Chicago's new summer heat index could increase to around 93 °F by the end of the century-similar to current summer conditions in Atlanta, Georgia.

City officials worry that intense summer heat could lead to uncomfortable conditions for residents, as well as reduced tourist attraction in summer months.

Extreme heat events often affect our most vulnerable populations first.

Trees, green roofs, and　vegetation can help reduce urban heat island effects by shading building surfaces, deflecting radiation from the sun, and releasing moisture into the atmosphere.

On this page:

This page contains technical information and references for state and local government officials working in the field of stormwater management.

How To

To reduce the urban heat island effect:

Build green infrastructure improvements into regular street upgrades and capital improvement projects to ensure continued investment in heat-reducing practices throughout your community.

Plant trees and other vegetation-Space in urban areas might be limited, but you can easily integrate small green infrastructure practices into grassy or barren areas, vacant lots, and street rights-of-way.

City officials in Louisville, Kentucky, recently awarded a $115,700 contract for a tree canopy assessment to help the city use trees to address urban heat, stormwater management, and other concerns. "Knowing where we lack canopy, down to the street and address level, will help our efforts exponentially," remarked Mayor Greg Fischer.

Make traditional water quality practices serve double duty by adding trees in or around roadside planters and other green infiltration-based practices to boost roadside cooling and shading.

Transform your community one project at a time by planting native, drought-tolerant shade trees and smaller plants such as shrubs, grasses, and groundcover wherever possible.

Build green roofs-Green roofs are an ideal heat island reduction strategy, providing both direct and ambient cooling effects. In addition, green roofs improve air quality by reducing the heat island effect and absorbing pollutants. Many communities offer tax credits for installing green roofs. Check your local government's website for opportunities. Examples of current programs include RiverSmart Rooftops Green Roof Rebate Program by the District of Columbia and Philadelphia's Green Roof Tax Credit Program

A number of tools are available to help you map the benefit of reducing heat-related stress on your community. For example, Impact Infrastructure Exit in conjunction with the Institute for Sustainable Infrastructure (ISI) Exit recently developed the Business Case Evaluator (BCE) for Stormwater Exit, a risk-based spreadsheet economic companion tool to ISI's Envision Sustainable Infrastructure Rating System. The tool includes estimates for the value of a comprehensive list of benefits, including reduced heat-related morality rates.

Resources

U.S. EPA Urban Heat Island Program -Clearinghouse for resources, tools, and educational materials on heat island effects, as well as a compendium of mitigation strategies.

EPA Region 5 Resource - Stormwater Trees -- A how-to technical memorandum for improving success of urban tree programs.

(90 pp, 849 K, Federal Funding Compendium for Urban Heat Adaptation (PDF)About PDF) Exit -The Georgetown Climate Center offers this compendium of federal funding opportunities related to urban heat island effects.

Adapting to Urban Heat-A Toolkit for Local Governments Exit -This page on the Georgetown Climate Center website provides an overview of the toolkit.

Reducing Urban Heat Islands: Compendium of Strategies-Green Roofs (PDF) (29 pp, 4MB, About PDF)-This EPA publication describes the causes and impacts of summertime urban heat islands and promotes strategies for lowering temperatures in U.S. communities.

iTree Exit -A U.S. Forest Service analysis tool for urban forest managers. It uses tree inventory data to quantify the dollar value of annual environmental benefits (e.g., energy conservation, air quality improvement, carbon dioxide reduction, stormwater control, and property value increase).

(52 pp, 2 MB, The Value of Green Infrastructure for Urban Climate Adaptation (PDF)About PDF) Exit -This Center for Clean Air Policy report evaluates the performance and benefits of a variety of green infrastructure strategies that assist with community climate resiliency, including ecoroofs, green alleys and streets, and urban forestry.

(76 pp, 2 MB, Green Works for Climate Resilience: A Guide to Community Planning for Climate Change (PDF)About PDF) Exit -This guide from the National Wildlife Federation provides an overview of nature-based approaches to address and plan for the effects of climate change, including planning resources related to mitigation of extreme heat.

References

Rosenzweig, C., W. Solecki, and R. Slosberg. 2006. Mitigating New York City's Heat Island with Urban Forestry, Living Roofs, and Light Surfaces. A Report to the New York State Energy Research and Development Authority (PDF). (156 pp, 4MB) .

Santamouris, M. 2014. Cooling the cities-A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Solar Energy 103:682-703.

Bowler, D. E., L. Buyung-Ali, T. M. Knight, and A. S. Pullin. 2010. Urban greening to cool towns and cities: A systematic review of the empirical evidence. Landscape and Urban Planning 97(3):147-155.

Hayhoe, K. Texas Tech University; D. Wuebbles, University of Illinois at Urbana-Champaign. 2008. Chicago Climate Change Action Plan - Climate Change and Chicago: Projections and Potential Impacts, Executive Summary. Exit.

Oliveira, S., H. Andrade, and T. Vaz. 2011. The cooling effect of green spaces as a contribution to the mitigation of urban heat: A case study in Lisbon. Building and Environment 46(11):2186-2194.

Bruggers, J. 2014, May 7. Study to examine Louisville's trees. The Courier-Journal. Exit.

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Reduce Urban Heat Island Effect | US EPA

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"Urban heat islands" occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. This effect increases energy costs (e.g., for air conditioning), air pollution levels, and heat-related illness and mortality. Climate change will likely lead to more frequent, more severe, and longer heat waves during summer months.

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ClimateBits | 2017

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Do hot neighborhoods affect everyone equally?    

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Do hot neighborhoods affect everyone equally?    

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5 Steps To Mitigate Urban Heat Island Effects

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5 Steps To Mitigate Urban Heat Island Effects

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5 Steps To Mitigate Urban Heat Island Effects

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Urban Heat Islands - Hot Times in the City

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Urban Heat Islands - Hot Times in the City