What Is the Heat Island Effect?

Have you ever noticed that urban areas often feel hotter than their more rural surroundings? The culprit? The heat island effect.

The heat island effect is caused by pavement, rooftops, and building surfaces in urban areas that absorb and reflect more of the sun’s heat. As a result, temperatures in cities and urban areas end up being higher than the surrounding rural areas. The relative scarcity of trees and other vegetation in cities also contributes to the heat island effect by reducing shade and cooling. Cities can be as much as 6 to 9 degrees warmer than outlying areas.

Increased heat causes several problems. For one, it's a health hazard. It also forces people to use more air conditioning, which leads to increases in greenhouse gas emissions.

These so-called “impervious surfaces” include roofs, roads, driveways, sidewalks, and patios. These surfaces replace vegetation and prevent water penetration, absorbing more heat as a result. Land development for buildings and roads for housing, commercial activity, industry, and transportation, create impervious surfaces. A national survey in 2006 estimated that there were 40,006 square miles of impervious surfaces in the U.S., an area slightly smaller than the state of Kentucky, representing a 4% increase since 2001.

This development has been accompanied by an increase in transportation and other infrastructure needed to serve an increasingly dispersed population. Roads alone represent a considerable portion of the built environment. In 2010, the U.S. had more than 4 million miles of roads owned and maintained by a public authority and open to public travel. Parking structure and parking lots are also significant components of our transportation infrastructure.

The EPA suggests that compact communities and buildings could help minimize developed area and potentially lessen the heat island effect. According to a study in Atlanta, a 25% reduction in impervious cover of a single-family lot was associated with a 16% reduction in surface heat island formation, and combined with a 25% reduction in landscaping area (i.e., smaller lot sizes), the surface heat island formation was reduced by 28%.

Building green infrastructure at the site scale, known as low-impact development, is another approach to help resolve this issue. For example, creating green roofs, parks, gardens and other green spaces will reduce heat absorption. The additional trees and shrubs can also filter air pollutants and improve air quality. These elements can also be incorporated into building codes or green building standards.

The Heat Island Effect is ultimately the consequence of a poorly-designed built environment. Incorporating more green spaces and limiting the amount of impervious surfaces in urban areas can help reduce the effect, while keeping our air clean and increasing our quality of life.

- Written by Jiin G. Park

**Jiin G. Park is an intern at Earth Day Network and a MA candidate in Environmental Conservation Education at New York University.