Encyclopedic Entry

In the summer, New York City can be 2-3 degrees hotter than rural areas nearby.

Photograph by Peggy Fahey, MyShot

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!

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.


Term Part of Speech Definition Encyclopedic Entry



to soak up.

air conditioning


system that cools the air.

air quality


measurement of pollutants and other harmful materials in the air.



having to do with water.



person who designs buildings or other large structures.

carbon dioxide


greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.



large settlement with a high population density.



having parts or molecules that are packed closely together.



construction or preparation of land for housing, industry, or agriculture.



capacity to do work.



person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

global warming


increase in the average temperature of the Earth's air and oceans.

Encyclopedic Entry: global warming

green roof


top of a residential or industrial building that is wholly or partially covered in vegetation.



having to do with factories or mechanical production.



to cover with material to prevent the escape of energy (such as heat) or sound.

landscape designer


person who studies and plans gardens, parks, and other "green spaces."



to stay longer than anticipated.

metropolitan area


region surrounding a central city and has at least 15 percent of its residents working in the central city.

native species


species that occur naturally in an area or habitat. Also called indigenous species.



chemical or other substance that harms a natural resource.



to lower or lessen.



available supply of materials, goods, or services. Resources can be natural or human.

rolling blackout


power outage, scheduled on purpose to conserve energy.

rural area


regions with low population density and large amounts of undeveloped land. Also called "the country."

Encyclopedic Entry: rural area



to disperse or distribute without a clear pattern.



very tall building.



to stretch beyond a reasonable or safe limit.




urban area


developed, densely populated area where most inhabitants have nonagricultural jobs.

Encyclopedic Entry: urban area

urban heat island


city area that is always warmer than the surrounding area.

Encyclopedic Entry: urban heat island

utility company


business that organizes and delivers public services such as electricity, water, or natural gas.

water quality


chemical, physical, and biological characteristics of water for a specific purpose such as drinking.


Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.


Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt


Tim Gunther
Mary Crooks, National Geographic Society


Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne


Caryl-Sue, National Geographic Society


Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

User Permissions

For information on user permissions, please read our Terms of Service.

If you have questions about licensing content on this page, please contact natgeocreative@ngs.org for more information and to obtain a license.

If you have questions about how to cite anything on our website in your project or classroom presentation, please visit our FAQ page.


Some media assets (videos, photos, audio recordings and PDFs) can be downloaded and used outside the National Geographic website according to the Terms of Service. If a media asset is downloadable, a download button appears in the lower right hand corner (download) of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.