About the Urban Heat Island Effect

During the summer, dark-colored rooftops, pavements and surfaces absorb and radiate heat from the sun. This phenomenon creates what is known as the Urban Heat Island effect, with urban temperatures 6 to 10 degrees Fahrenheit higher than rural temperatures. Anyone who has experienced the change in temperature upon moving from an asphalt parking lot to a lush park on a hot summer day has experienced the Urban Heat Island effect.

In addition to dark surfaces, a second contributing factor to the Urban Heat Island effect is the decreased amount of evaporation in city environments. Evaporation is a cooling process because energy (heat) is used to change liquid water into vapor that rises up off a surface. Our bodies use this process when we are hot by producing sweat; the evaporation of the sweat off our skin cools us. Evaporation is a natural cooling process the rate of which increases as temperature increases, thereby moderating summer temperatures. However, in the city, much less water is present at the surface because pavement and buildings do not hold water like soil and plants do. When it rains, most water is collected in gutters and diverted into sewers, where it is carried away from the city, leaving the city without an important cooling tool.

The higher temperatures created by the Urban Heat Island effect are not only uncomfortable, they also have implications for air pollution. Smog is created by photochemical reactions of pollutants in the air. These reactions are more likely to occur and intensify at higher temperatures. The result is an accumulation of smog, which can threaten human health and damage the natural environment.

In addition to being uncomfortable and a potential health hazard, the Urban Heat Island effect costs consumers money because more energy is required to cool buildings. An increase in energy consumption also increases the amount of pollution emitted into the air. Power plants must work overtime to keep up with the increased demand in energy.

There are actions that individuals and cities can take to lessen the Urban Heat Island effect. The City of Chicago has adopted a variety of strategies to help cool our city, save money and reduce air pollution. We encourage residents, businesses and builders to do the same. For more information, see "Reducing the Urban Heat Island Effect".