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.

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?

• 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.

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