Basic Information

Heat Islands

What Is a Heat Island?

The term "heat island" refers to urban air and surface temperatures that are higher than nearby rural areas. Many U.S. cities and suburbs have air temperatures up to 10°F (5.6°C) warmer than the surrounding natural land cover.

Graphic depicting a typical rise in temperature from rural areas to an urban center.

The heat island sketch pictured here shows a city's heat island profile. It demonstrates how urban temperatures are typically lower at the urban-rural border than in dense downtown areas. The graphic also show how parks, open land, and bodies of water can create cooler areas.

For more basic information about heat islands, watch two short video segments that EPA developed in partnership with The Weather Channel cable television network.

How Do Heat Islands Form?

Heat islands form as cities replace natural land cover with pavement, buildings, and other infrastructure. These changes contribute to higher urban temperatures in a number of ways:

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.

In addition to these factors, heat island intensities depend on an area's weather and climate, proximity to water bodies, and topography. Measuring heat islands can help determine how these factors influence the heat island effect.

When Do Heat Islands Form?

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.

Can Heat Islands Provide Wintertime Benefits?

In the wintertime, some cities in cold climates may benefit from the warming effect of heat islands. Warmer temperatures can reduce heating energy needs and may help melt ice and snow on roads. In the summertime, however, the same city will experience the negative effects of heat islands: increased levels of air conditioning demand, air pollution, greenhouse gas emissions, and heat-related illness and mortality, as well as reduced thermal comfort.

In general, the harmful impacts from summertime heat islands are greater than the wintertime benefits, and most heat island reduction strategies can reduce summertime heat islands without eliminating wintertime benefits.

Are Heat Islands and Global Warming Related?

Heat islands describe local-scale temperature differences, generally between urban and rural areas. In contrast, global warming refers to a gradual rise of the earth's surface temperature.

While they are distinct phenomena, summertime heat islands may contribute to global warming by increasing demand for air conditioning, which results in additional power plant emissions of heat-trapping greenhouse gases. Strategies to reduce heat islands, therefore, can also reduce the emissions that contribute to global warming.

The heat island effect can also complicate studies of long-term trends. By accurately measuring heat islands, scientists can remove the heat island effect from global temperature records.

EPA's global warming web site describes the greenhouse effect and our changing climate.

What is EPA's HIRI Program Doing to Reduce Urban Heat Islands?

EPA's Heat Island Reduction Initiative (HIRI), works with community groups, public officials, industry representatives, researchers, and others to identify opportunities to implement heat island strategies.

HIRI supports research to better understand the impacts that heat island reduction strategies have on urban meterology, air quality, energy demand, and human health. The program translates this research into outreach materials, tools, and guidance that provide communities with information to develop programs, policies, codes, and ordinances to implement heat island reduction strategies.

Visit the research page of this web site for more information on HIRI-supported initiatives.