Urban Heat Island

Urban areas often experience warmer temperatures than their surrounding rural areas. This phenomenon, called the urban heat island (UHI) effect, has been observed in numerous settlements around the world, making it one of best-documented examples of inadvertent climate modification. Heat islands occur in the air, at the surface, and in the ground beneath cities. Atmospheric heat islands may extend to considerable distances aloft, although UHIs occurring below roof level in the urban canopy layer have received the most attention in the literature. As an increasing proportion of the world's population lives in urban areas, cities grow, and their impact on environmental, biophysical, and atmospheric processes increases. This entry provides a brief overview of UHI impacts, describes the physical causes of UHIs, and closes with a brief synopsis of urban climate research.

May 19, 1998: Hot rooftops shine brightly in a fresh, false-color image of Baton Rouge, Louisiana, taken as part of the Urban Heat Island Pilot Project (UHIPP) conducted by NASA and other agencies. It clearly demonstrates the principle behind UHIPP—that the differences in cooling and heating between the natural and humanmade surfaces can affect city temperatures.

Source: NASA/Marshall Space Flight Center.

Urban heat islands directly affect a number of human and natural systems. UHIs also generate indirect impacts through altering mesoscale circulations such as thunderstorms and urban/country breezes. Urban gardeners are well aware that the UHI alters flowering dates and extends the growing season in cities. Urban warmth can be favorable for many species of plants and animals, particularly during the cool seasons. Conversely, elevated urban temperatures often negatively [Page 2948]affect human comfort and health during warm seasons. Sensitive demographic cohorts such as the elderly and infirm are at particularly high risk of heat-related illnesses and mortality. Energy demands are also affected by the UHI. In warm climates, the higher number of cooling degree days occurring within cities further increases cooling costs. In cool climates, however, energy savings from reduced heating degree days may offset or surpass the increased cooling bills. Even climate science is affected by the UHI since artificially high temperatures can introduce bias into long-term temperature records. If land use/land cover is changed in the vicinity of a weather station, localized climatic changes could appear in the station's record and be misinterpreted as a regional or even global signal of climate change. Removing this urban bias requires sophisticated statistical techniques. The influence of urban areas on large-scale temperature trends continues to attract attention among climatologists.

UHIs are not limited to large cities; small towns generate heat islands too. In fact, any place where artificial land cover has replaced natural land cover could have a climate that is different from its surroundings. Replacing natural land covers, such as trees and grass, with artificial land covers, such as concrete and asphalt, alters the local energy balance. First of all, more incoming energy is retained in urban areas than in rural areas. Many urban surfaces have a low albedo (reflectivity) and often absorb more solar energy than the natural land covers they have replaced. In addition, the geometry of street canyons causes multiple reflections within the canyon, allowing multiple opportunities for building materials to absorb energy. Along with the aforementioned characteristics, the physical and thermal properties of construction materials are also different from those natural materials. Many building materials are very effective at retaining absorbed solar energy and radiating heat into the surrounding environment for hours after sunset. By comparison, rural areas generally cool much more quickly. Therefore, differences in urban and rural cooling rates after sunset often generate the UHI. Additionally, maximum nocturnal UHI development typically occurs on calm, clear nights. Calm winds limit the mixing of urban and rural air, and clear skies promote rapid nocturnal cooling through the loss of longwave radiation.

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