Heat islands
Heat islands refer to dome-shaped areas of warm air that accumulate over urban areas, significantly elevating temperatures compared to surrounding rural regions. This phenomenon results from several factors, including the heat-retaining properties of urban materials like concrete and asphalt, complex urban structures that facilitate energy exchanges, and artificial heat generation primarily during colder months. Additionally, urban heat islands are influenced by rapid drainage systems that lower humidity and by air pollution, which can trap heat within the city.
The temperature differences in heat islands can be most pronounced during winter nights, leading to longer growing seasons and altered freeze dates compared to adjacent nonurban areas. Pollution levels tend to be higher within these heat islands due to local air circulation patterns, which can also diminish visibility and exacerbate health issues. While the impact of heat islands on precipitation is still debated, there is some evidence that cities experience slightly more rainfall and an increased frequency of thunderstorms and fog. Technological advancements, such as satellite temperature measurements, have enhanced understanding and mapping of heat islands, providing valuable insights for urban planning and environmental management.
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Heat islands
DEFINITION: Dome-shaped areas of warm air that form over cities
Heat islands increase temperatures, concentrate pollution, and may affect precipitation in urban environments.
Five factors are responsible for the formation of urban heat islands: urban fabric, or the rocklike material of a city’s buildings and streets, which conducts heat three times faster than nonurban sandy soil; city structure, a complex web of multiple reflections and energy exchanges; artificial heat production, which occurs mainly in winter when artificial heating is highest; urban water balance, marked by rapid drainage and reduced humidity; and urban air pollution, which retains heat within the dome.
The impacts of heat islands on urban environments are many. The retention of heat leads to temperatures that are several degrees higher than those in surrounding nonurban outskirts. These temperature differences tend to be greatest in winter and at night, but heat islands also exist during summer. Higher temperatures lead to longer freeze-free seasons in cities, with first freezes in the fall occurring about one month later and last freezes in the spring occurring about one month earlier on average than in the surrounding rural areas.
Pollution is greater in urban areas because of the local circulation produced within cities and the polluting industries situated in them. The local circulation of a city’s results from warm air rising near the central business district, spreading out at the top of the dome, descending at the outer edges, and flowing back into the city center. Pollution tends to attenuate incoming shortwave and reduce visibility. Pollution levels tend to be highest in winter, when the sun angle is low, and on workdays. Heat islands are associated with disasters such as those that occurred in London, England, in 1952; in Donora, Pennsylvania, in 1948; and in the Meuse Valley of Belgium in 1930. Each of these events occurred during the cold season.
The impacts of heat islands on precipitation remain in question. Some researchers have found that greater precipitation amounts over cities are largely the result of air rising in the city center and the greater abundance of condensation nuclei over the city. Others argue that the number of condensation nuclei is so vast that cloud drops are too tiny to grow to raindrop size, thus reducing the amount of rain. However, it is generally accepted that the number of rainy days in cities is slightly higher than the number in rural areas. Most urban climatologists have reported higher frequencies of hail and thunderstorms in cities and have noted that fog is more prevalent there than in outlying areas. The heat island effect generally produces less snow and more rain during storms. Humidity, both relative and absolute, tends to be slightly lower in urban environments than in areas outside cities. Technological advances have greatly increased scientists' understanding of heat islands. Satellite measurements of air temperature have enabled them to create maps of the heat island effect throughout urban landscapes. Scientists have used distributed monitor networks to create very detailed mappings.
Bibliography
Aguado, Edward, and James E. Burt. “Human Effects: Air Pollution and Heat Islands.” In Understanding Weather and Climate. 5th ed. Upper Saddle River, N.J.: Pearson Education, 2010.
Alberti, Marina. Advances in Urban Ecology: Integrating Humans and Ecological Processes in Urban Ecosystems. New York: Springer Science, 2008.
Gartland, Lisa. Heat Islands: Understanding and Mitigating Heat in Urban Areas. Sterling, Va.: Earthscan, 2008.
"Heat Island Effect." US Environmental Protection Agency (EPA), 6 June 2024, www.epa.gov/heatislands. Accessed 17 June 2024.
Pine, Joshua, et al. "Urban Heat Island Effect Solutions and Funding." National League of Cities, 13 Feb. 2023, www.nlc.org/article/2023/02/13/urban-heat-island-effect-solutions-and-funding/. Accessed 17 June 2024.