EPA: Urban Heat Islands

The Environmental Protection Agency describes the effects of development on local urban temperatures. Note the dramatic moderating effect of an urban park.

What Is a Heat Island?

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

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.

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
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
  • 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

See also:

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Total Parcel Size: 5.49 [acres]
Total Number of Units Proposed: 31
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Cooling Our Cities
[PDF]: A fact sheet on tree planting as a way to save money and