As solar energy strikes the earth's surface, it can have many effects. In rural areas, a large amount of solar energy is expended on evaporating water. Plants facilitate the cycle by transpiring water through their leaves, which then cools the surrounding air. In urban areas, impervious cover such as pavement and roofing produces a different process. Dark materials absorb solar energy and then release it back to the surrounding air as heat. These conditions cause temperatures in some urban areas to be much hotter than rural areas. This phenomenon is known as "urban heat island."
 

Studies have shown that:

Temperatures in some urban areas of the United States can be eight degrees Fahrenheit (°F) hotter than surrounding areas (1).

Heat accelerates chemical reactions in the atmosphere, leading to higher ozone concentrations and greater potential risk to human health.

Three to eight percent of urban electricity demand is used to compensate for increased temperatures from urban heat island effect.

Landsat TM thermal infrared image

The Landsat Thematic Mapper (TM) thermal infrared satellite image on the left offers a regional perspective of the heat island phenomenon in Chicago, Illinois. In the image, radiant temperatures are communicated as tonal variations: lighter tones represent "warmer" features, darker tones represent "cooler" features.

Subsets 1 and 2: Thermal infrared and vegetation subsets
Thermal values increase from rural to urban areas as vegetation is replaced by pavement and roofing. Landsat TM images can be analyzed to identify these conditions. Subsets 1 and 2 illustrate the relationship between impervious cover, vegetation, and radiant temperatures for a residential neighborhood and forest preserve. The forest preserve has lower air temperatures than the residential neighborhood due to the cooling effects of vegetation and the absence of dark impervious surfaces.

Subset 1 - Residential neighborhood
Urban vegetation conditions (left) with associated thermal conditions (right). White areas on the land cover map have little or no vegetation, dark green patches are urban parks.

Subset 2 - Forest preserve
Forest preserve (dark green) on left with associated thermal conditions (right).