Urban Heat Islands

The table below summarises some of the differences in weather conditions between urban areas and rural locations.

A marked difference in air temperature is one of the most important contrasts between urban and rural areas shown in the table above. For instance a study in 1965 showed that under clear skies and light winds, temperatures in central London during the spring reached a minimum of 11°C, whereas in the suburbs they dropped to 5°C.

Indeed, the term urban heat island is used to describe the dome of warm air that frequently builds up over towns and cities.

The formation of a heat island is the result of the interaction of the following factors:

• the release (and reflection) of heat from industrial and domestic buildings;
• the absorption by concrete, brick and tarmac of heat during the day, and its release into the lower atmosphere at night;
• the reflection of solar radiation by glass buildings and windows. The central business districts of some urban areas can therefore have quite high albedo rates (proportion of light reflected);
• the emission of hygroscopic pollutants from cars and heavy industry act as condensation nuclei, leading to the formation of cloud and smog, which can trap radiation. In some cases, a pollution dome can also build up;
• recent research on London's heat island has shown that the pollution domes can also filter incoming solar radiation, thereby reducing the build up of heat during the day. At night, the dome may trap some of the heat from the day, so these domes might be reducing the sharp differences between urban and rural areas;
• the relative absence of water in urban areas means that less energy is used for evapotranspiration and more is available to heat the lower atmosphere;
• the absence of strong winds to both disperse the heat and bring in cooler air from rural and suburban areas. Indeed, urban heat islands are often most clearly defined on calm summer evenings, often under blocking anticyclones.

The precise nature of the heat island varies from urban area to urban area, and it depends on the presence of large areas of open space, rivers, the distribution of industries and the density and height of buildings. In general, the temperatures are highest in the central areas and gradually decline towards the suburbs. In some cities, a temperature cliff occurs on the edge of town.