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https://www.nationalgeographic.org/encyclopedia/urban-heat-island/

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An urban heat island, or UHI, is a metropolitan area that's a lot warmer than the rural areas surrounding it

Subjects

Earth Science, Meteorology, Geography, Human Geography, Physical Geography

An urban heat island, or UHI, is a metropolitan area that's a lot warmer than the rural areas surrounding it. Heat is created by energy from all the people, cars, buses, and trains in big cities like New York, Paris, and London. Urban heat islands are created in areas like these: places that have lots of activity and lots of people.

There are many reasons for UHIs. When houses, shops, and industrial buildings are constructed close together, it can create a UHI. Building materials are usually very good at insulating, or holding in heat. This insulation makes the areas around buildings warmer.

"Waste heat" also contributes to a UHI. People and their tools, such as cars and factories, are always burning off energy, whether they're jogging, driving, or just living their day-to-day lives. The energy people burn off usually escapes in the form of heat. And if there are a lot of people in one area, that's a lot of heat.

Urban areas are densely populated, meaning there are a lot of people in a small space. Urban areas are also densely constructed, meaning buildings are constructed very close together. When there is no more room for an urban area to expand, engineers build upward, creating skyscrapers. All this construction means waste heat-and heat that escapes insulation has nowhere to go. It lingers in and between buildings in the UHI.

Nighttime temperatures in UHIs remain high. This is because buildings, sidewalks, and parking lots block heat coming from the ground from rising into the cold night sky. Because the heat is trapped on lower levels, the temperature is warmer.

Urban heat islands can have worse air and water quality than their rural neighbors. UHIs often have lower air quality because there are more pollutants (waste products from vehicles, industry, and people) being pumped into the air. These pollutants are blocked from scattering and becoming less toxic by the urban landscape: buildings, roads, sidewalks, and parking lots.

Water quality also suffers. When warm water from the UHI ends up flowing into local streams, it stresses the native species that have adapted to life in a cooler aquatic environment.

Scientists are studying how urban heat islands might contribute to global warming, the most recent climate change pattern that includes the gradual warming of the Earth's temperature.

When it's really hot, many of us run straight to the fan or the air conditioning. This is especially true in urban areas that suffer from urban heat island effects. UHIs contribute to energy demands in the summer, straining energy resources. UHIs are often subject to "rolling blackouts," or power outages. Utility companies start rolling blackouts when they do not have enough energy to meet their customers' demands. The energy used in electric fans and air conditioning ends up contributing to an even hotter UHI.

Because of these negative effects, scientists say city dwellers, architects, and designers all have to work to reduce people's impact on urban areas. Using green roofs, which are roofs of buildings covered in plants, helps cool things down. Plants absorb carbon dioxide, a leading pollutant. They also reduce the heat of the surrounding areas. Using lighter-colored materials on buildings helps, too. Light colors reflect more sunlight and trap less heat.

Fast Fact

Turning Up the Heat
The Heat Island Group says that the urban heat island around Los Angeles, California, costs the city $100 million a year in energy!

Media Credits

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Illustrators

Mary Crooks, National Geographic Society

Editors

Jeannie Evers, Emdash Editing, Emdash Editing

Producer

National Geographic Society

other

Last Updated

December 15, 2023

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An urban heat island, or UHI, is a metropolitan area that's a lot warmer than the rural areas surrounding it

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https://climatekids.nasa.gov/heat-islands/

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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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https://www.sciencedirect.com/science/article/pii/S1877705816332039

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Abstract

Urban heat island (UHI) effect is a kind of heat accumulation phenomenon within urban area due to urban construction and human activities. It is recognized as the most evident characteristic of urban climate. The increase of land surface temperature caused by UHI effect will definitely influence material flow and energy flow in urban ecological systems, as well as alter their structure and functions, exerting a series of ecological and environmental effects on urban climates, urban hydrologic situations, soil properties, atmospheric environment, biological habits, material cycles, energy metabolism and residents' health. Through the improvement of energy efficiency, urban landscape optimization, green roof construction, high reflectivity material utilization and green land cultivation, UHI effects could be significantly mitigated. Based on remote sensing technology and numerical simulation methods, research on the ecological and environmental effects of UHI has been multi-scaled conducted, providing theoretical reference for the improvement of urban ecological environment and realization of urban sustainable development.

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Urban heat island (UHI) effect is a kind of heat accumulation phenomenon within urban area due to urban construction and human activities. It is recognized as the most evident characteristic of urban climate.

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https://www.epa.gov/heatislands
Heat islands are urbanized areas that experience higher temperatures than outlying areas. Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. Daytime temperatures in urban areas are about 1–7°F higher than temperatures in outlying areas and nighttime temperatures are about 2-5°F higher.

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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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Electric vehicles have gotten a lot of support because they have the potential to reduce air pollution and greenhouse gas emissions. Now it seems these cars may have another benefit-cooler cities.

A conventional gasoline engine is incredibly inefficient. Most of the energy produced by the tiny explosions inside is actually lost as heat. The engine that runs an electric car, by contrast, is much more efficient and loses far less energy to heat. The difference is enough that if all the cars in Beijing were replaced with electric versions, the city would be almost 2 degrees Fahrenheit cooler in summer, researchers conclude today in Scientific Reports.

This is especially important in metropolitan regions that experience the urban heat island effect, when cities are hotter than the surrounding countryside because all those buildings and concrete interfere with how efficiently the region can release heat into the air. "The stronger the urban heat island effect, the greater the benefits," says study leader Canbing Li of Hunan University and Michigan State University.

Li and his colleagues investigated how switching to electric vehicles would affect the urban heat island of Beijing. In that city, which has experienced temperatures as high as 122 degrees in past summers, half of the excess heat comes from vehicles and building air conditioners. In 2012 in Beijing, an electric vehicle emitted on average only 19.8 percent as much heat as a conventional vehicle, the researchers found. Some heat would still be generated by the power plants that produce energy for electric cars, though that didn't matter so much since these sites are usually far from city centers. But there would also be a feedback when it came to air conditioning-lower heat emissions from vehicles would reduce city temperatures, so people wouldn't have to use so much AC.

Overall, replacing conventional gas-guzzlers with electric cars would save some 14.4 million kilowatts of energy each day and reduce carbon dioxide emissions by nearly 12,000 tons daily, the researchers calculate. And instead of Beijing being 5.4 degrees warmer than the surrounding countryside, it would be only 3.7 degrees hotter. Li says he expects that other cities would see similar benefits.

That doesn't mean that everyone should run out and buy a Tesla. Even Li doesn't intend to buy an electric vehicle until his car reaches the end of its service life. And he and his colleagues note that there is an increasingly heated debate on whether the replacement of conventional vehicles by electric ones should be delayed or accelerated, because the manufacturing processes for new technologies are more costly and cause more pollution than those for conventional cars.

In addition, the benefits for battling climate change and reducing pollution aren't all that clear at this time. There are challenges to switching from engines and furnaces that burn fossil fuels to ones that run on electricity, such as the environmental impacts of battery disposal and costs, University of Toronto civil engineer Christopher Kennedy noted in a recent commentary in Nature. The root source of energy for a vehicle's batteries makes a huge difference. A study published in Environmental Science & Technology in 2011, for instance, concluded that electric cars in China actually generated more harmful air pollution than gasoline cars because the country's electricity came largely from coal-fired power plants.

Kennedy notes that greenhouse gas emissions also "are only reduced under electrification strategies if the displaced fossil fuels are replaced by electricity of suitably low carbon intensity." In other words, the electric car scheme only works for lowering pollution and preventing climate change if the vehicles draw their energy from renewable sources like wind and solar.

The reduction in summertime city temperatures, though, would be a nice benefit.

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Electric vehicles have gotten a lot of support because they have the potential to reduce air pollution and greenhouse gas emissions. Now it seems these cars may have another benefit-cooler cities. A conventional gasoline engine is incredibly inefficient. Most of the energy produced by the tiny explosions inside is actually lost as heat.

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https://www.newscientist.com/article/dn27195-electric-cars-are-so-cool-they-may-make-it-colder/

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There's more than one way to cut emissions. Swapping conventional cars for electric ones might cut emissions from cars, but it may also help cities like Beijing beat the heat, further lowering emissions of carbon dioxide by cutting the use of air conditioning.

The role of electric cars in combating climate change is hotly debated, because although they might emit less greenhouse gas directly, they are more expensive and cause more pollution than conventional cars during the manufacturing process. But it seems there may be hidden benefits to them.

Electric cars emit less than 20 per cent of the heat that conventional cars do, thus contributing less to the phenomenon of cities being warmer than the surrounding areas, known as the urban heat island effect.

If this lowers temperatures, use of air conditioning would also drop, and so would the energy used to power it, argues a team led by Canbing Li from Michigan State University in East Lansing.

Cooler summer

Using summer 2012 in the Chinese capital of Beijing as an example, the team estimate that replacing conventional cars with electric ones could have reduced the heat by nearly 1 °C. That in turn would result in a reduction in air conditioning use in the city, leading to a drop of 10,686 tonnes in daily CO ₂ emissions.

"I was surprised by the level of decreased energy consumption of air-conditioning," says Li.

The team says that there are several factors contributing to the urban heat island, and not all have been addressed in the study. This makes some people sceptical about the estimates.

"Investigating a mitigation strategy is much more complicated than just considering the heat flux," says Ali Gholizadeh Touchaei of the Heat Island Group at Concordia University in Montreal, Canada. "The sophisticated relation between moisture balance, effect of wind, and interactions of buildings and atmosphere should be accounted for."

And Oscar van Vliet of the Swiss Federal Institute of Technology in Zurich says that our heat emissions are only partly responsible for the heat island effect, it is also down to the city retaining that heat thanks to tarmac surfaces, which don't cool down quickly.

Cars of the future

Regardless of how big any cooling effect might be, the climate effects of urban transport merits further study with more sophisticated models, say other researchers.

"It is an interesting study, as the issue of heat generated by vehicles isn't something that has been thought about in great detail," says Timothy Johnson of Duke University in Durham, North Carolina.

It could become an ever bigger influence as electric cars catch on. Johnson expects the vehicles to dominate the transportation market in a few more decades once they overcome the technical challenges of driving range and battery charging infrastructure.

Unlike with conventional cars, which depend on non-renewable fuels, the disadvantages of electric cars can be overcome, says Li.

Journal reference&colon; Scientific Reports, DOI&colon; 10.1038/srep09213

Clarification, 20 April 2015&colon; The amount of heat generated by electric cars has been clarified since this article was first published.

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Electric cars are so cool they may make it colder

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There's more than one way to cut emissions. Swapping conventional cars for electric ones might cut emissions from cars, but it may also help cities like Beijing beat the heat, further lowering emissions of carbon dioxide by cutting the use of air conditioning.

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UCAR of Science Education

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Temperature (top) and vegetation (bottom) for locations in and around New York City via NASA satellite. Do you notice that the pattern of purple and white in the top image look similar to the pattern of green and white in the bottom image? Where vegetation

is dense (dark green), temperature is cooler (dark purple). Where vegetation is sparse, temperature is hot. (Image: NASA)

An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. According to the EPA, many U.S. cities have air temperatures up to 10°F (5.6°C) warmer than the surrounding natural land cover. This temperature difference usually is larger at night than during the day and larger in winter than in summer, and is most apparent when winds are weak. The main causes are changes in the land surface by urban development along with waste heat generated by energy use. As population centers grow, they tend to change greater areas of land which then undergo a corresponding increase in average temperature.

How Do Heat Islands Form?

Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb-rather than reflect-the sun's heat, causing surface temperatures and overall ambient temperatures to rise. Displacing trees and vegetation minimizes the natural cooling effects of shading and evaporation of water from soil and leaves (evapotranspiration). Tall buildings and narrow streets can heat air trapped between them and reduce air flow. Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect. Heat islands can occur year-round during the day or night. Urban-rural temperature differences are often largest during calm, clear evenings. This is because rural areas cool off faster at night than cities, which retain much of the heat stored in roads, buildings, and other structures. As a result, the largest urban-rural temperature difference, or maximum heat island effect, is often three to five hours after sunset. Other contributing factors to the heat island effect include:

Size and shape of cities-aerodynamically, cities have a very different shape than rural areas. Tall buildings act as obstacles and reduce wind speeds.

Urban deserts-cities can be thought of as virtual deserts with almost no vegetation and materials that are almost completely impermeable to rain. This combination leads to a lack of evapotranspiration which increases sensible heat.

Urban canyons-the tall canyons formed by city buildings trap radiant energy in their walls. Comparisons of this "canyon effect" in European and North American cities suggest that areas with denser and taller buildings will more rapidly develop heat islands.

Humidity effects-although there is little difference in the amount of water that cities and countrysides retain in their atmospheres (absolute humidity), the higher urban temperatures effectively lowers the relative humidity (since warm air can hold more water than cold air).

Urban haze-The haze of air pollution that hangs over many cities can act as a miniature greenhouse layer, preventing outgoing thermal radiation (heat) from escaping from urban areas.

Anthropogenic heat-The release of heat from the burning of fossil fuels can also raise urban temperatures. On a typical winter day, Manhattan releases four times more energy from burning fossil fuels than the amount of energy that comes into the urban area from the Sun.

Are Heat Islands and Climate Change Related?

Heat islands describe local temperature differences, generally between urban and rural areas. In contrast, warming caused by climate change refers to rise in the Earth's surface temperature globally. Scientists account for urban heat when they are taking Earth's temperature so that it doesn't affect measurements of global climate.

There is a myth that urban heat is to blame for climate change. This is not the case. Both urban and rural environments around the world are warming because there are more greenhouse gases in the atmosphere. In fact, some of the areas of the world that have warmed the most are located in the Arctic, far from cities.

Urban areas are more vulnerable to heat, however, because the amount of warming caused by global climate change is compounded by the urban heat island effect. That means that people who live in cities are going to face higher temperatures and stronger heat waves in the future as climate warms. More than half the global population lives in urban areas today and by the year 2050, the percentage of urban dwellers worldwide is expected to reach 70%, so the problem of urban heat islands will continue to grow.

Urban heat islands may also contribute to climate change by increasing the demand for air conditioning during heat waves. When the energy for that air conditioning comes from the burning of fossil fuels, this results in emissions of heat-trapping greenhouse gases.

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Temperature (top) and vegetation (bottom) for locations in and around New York City via NASA satellite. Do you notice that the pattern of purple and white in the top image look similar to the pattern of green and white in the bottom image? Where vegetationis dense (dark green), temperature is cooler (dark purple).

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http://www.patarnott.com/pdf/Oake1982_UHI.pdf

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https://www.epa.gov/heatislands

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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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How do different dark and light materials contribute to the UHIE? 

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How do different dark and light materials contribute to the UHIE? 

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Darker colors absorb more light and will therefore have higher temperatures. We believe this will be the case for all surfaces accept for dirt and grass. Our team believes these two will be cooler, grass being the coolest because of radiation energy being used for photosynthesis and evapotranspiration.

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Darker colors absorb more light and will therefore have higher temperatures. We believe this will be the case for all surfaces accept for dirt and grass. Our team believes these two will be cooler, grass being the coolest because of radiation energy being used for photosynthesis and evapotranspiration.

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Do you know if any research has been done on the reflective surfaces of cars contributing to UHIE?

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Do you know if any research has been done on the reflective surfaces of cars contributing to UHIE?

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Is it possible to use a satellite image to quantify percentage of types of surfaces?  Or percentages of light vs. dark roofs?

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Is it possible to use a satellite image to quantify percentage of types of surfaces?  Or percentages of light vs. dark roofs?

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Why might the tar, which is black, be painted metallic silver?

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Where is a safe location to reach and take surface temperature measurements of these tiles?

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Maybe the shed /garage at the school?

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My roof is metal, it is brown. -Mrs. Smith

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Avoid measurements from the hood in case the engine is warm.
Should we take measurements of black and white cars or all colors?

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Observations of the Urban Heat Island Effect in New York City

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Abstract

Differences in the temperature fields through the lowest 700 m of the atmosphere in and around New York City during the hours near sunrise are analyzed. Data were obtained by an instrumented helicop…

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Assessment of the Urban Heat Island Effect Through the Use of Satellite Data

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Abstract A recent NASA satellite is obtaining high spatial resolution thermal infrared data at times of day appropriate for, the study of the urban heat island effect. Quantitative estimates of the extent and intensity of urban surface heating are obtained by analysis of digital data acquired over the New York City-New England area.

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Questioned Bob about the surface of the school roof

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Questioned Bob about the surface of the school roof

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He said the material used is a rubber membrane that is water proof and painted silver to specifically reflect sunlight radiation.

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Is Urban heat a problem in cooler climates and winter?

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Is Urban heat a problem in cooler climates and winter?

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IRTs
iPads for GLOBE App protocols
Clipboard with data sheet and pencil.
Air Temperature Thermometer
Shingles and other roofing materials
A Ladder
Cars of various colors
Bucket Truck

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1. Take materials outside and take the air temperature. Record date and time. Observe weather conditions.

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1. Take materials outside and take the air temperature. Record date and time. Observe weather conditions.

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2. Use a ladder to take surface temperature measurements of garage roofs, FAC, school (with the help of a bucket truck), asphalt, concrete, gravel, grass and dirt.

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2. Use a ladder to take surface temperature measurements of garage roofs, FAC, school (with the help of a bucket truck), asphalt, concrete, gravel, grass and dirt.

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3. Use the stairs to gain access to the roof for membrane surface measurements.

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3. Use the stairs to gain access to the roof for membrane surface measurements.

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4. Take surface temperature measurements of all cars in staff parking lot.  

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4. Take surface temperature measurements of all cars in staff parking lot.  

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All team members researched topic, defined key words, collected surface temperature data, collaborated on writing introduction and methods.

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All team members researched topic, defined key words, collected surface temperature data, collaborated on writing introduction and methods.

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Conner did an exceptional job analyzing his professional peer reviewed article.

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Conner did an exceptional job analyzing his professional peer reviewed article.

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Should we separate the car data from other surfaces (2 separate projects)?

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Should we separate the car data from other surfaces (2 separate projects)?