What is the Heat Island Effect?
On hot summer days, urban air can be up to 10°F hotter than the surrounding countryside. Not to be confused with global climate change, scientists call this phenomenon the "heat island effect." Heat islands form as cities replace natural land cover with pavement, buildings, and other infrastructure.
Increased urban temperatures can affect public health, the environment, and the amount of energy that consumers use for summertime cooling.
- Public Health: Heat islands can amplify extreme hot weather events, which can cause heat stroke and lead to physiological disruption, organ damage, and even death – especially in vulnerable populations such as the elderly.
- The Environment: Summertime heat islands increase energy demand for air conditioning, raising power plant emissions of harmful pollutants. Higher temperatures also accelerate the chemical reaction that produces ground-level ozone, or smog. This threatens public health, the environment, and for some communities may make it harder to meet federal air quality goals.
- Energy Use: Because homes and buildings absorb the sun’s energy, heat islands can increase the demand for summertime cooling and raise energy expenditures. For every 1° F (0.6° C) increase in summertime temperature, peak utility loads in medium and large cities increase by an estimated 1.5–2.0 percent.
Cities in cold climates may actually benefit from the wintertime 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 may experience the negative effects of heat islands.
Fortunately, there are a number of steps that communities can take to lessen the impacts of heat islands. These "heat island reduction strategies" include:
- Installing cool or vegetated green roofs
- Planting trees and vegetation
- Switching to cool paving materials
Visit EPA's Heat Island Site for more information on what local governments and citizens can do to address the problem of rising urban temperatures.