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An urban heat island (UHI) is a city or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon.〔Luke Howard, ''The climate of London, deduced from Meteorological observations, made at different places in the neighbourhood of the metropolis'', 2 vol., London, 1818-20〕 The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The main cause of the urban heat island effect is from the modification of land surfaces.〔United States. Environmental Protection Agency. Reducing Urban Heat Islands: Compendium of Strategies - Urban Heat Island Basics. By EPA. N.p.: n.p., n.d. Print〕 Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area and increase its average temperature. The less-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area. Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increases the length of growing seasons, and decreases the occurrence of weak tornadoes. The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams and put stress on their ecosystems. Not all cities have a distinct urban heat island. Mitigation of the urban heat island effect can be accomplished through the use of green roofs and the use of lighter-colored surfaces in urban areas, which reflect more sunlight and absorb less heat. There are concerns raised about possible contribution from urban heat islands to global warming. Research on China〔Huang, Q., & Lu, Y. (2015). The Effect of Urban Heat Island on Climate Warming in the Yangtze River Delta Urban Agglomeration in China. International Journal of Environmental Research and Public Health, 12(8), 8773-8789.〕〔Ren, Y., Parker, D., Ren, G., & Dunn, R. (2015). Tempo-spatial characteristics of sub-daily temperature trends in mainland China. Climate Dynamics, 1-12.〕 and India〔Joshi, R., Raval, H., Pathak, M., Prajapati, S., Patel, A., Singh, V., & Kalubarme, M. H. (2015). Urban Heat Island Characterization and Isotherm Mapping Using Geo-Informatics Technology in Ahmedabad City, Gujarat State, India. International Journal of Geosciences, 6(03), 274.〕 indicates that urban heat island effect contributes to climate warming by about 30%.〔Z.-C. Zhao (2011) Impacts of urbanization on climate change , 10,000 Scientific Difficult Problems: Earth Science (in Chinese), 10,000 scientific difficult problems Earth Science Committee Eds, 2011, Science Press, pp. 843–846〕 On the other hand, one 1999 comparison between urban and rural areas proposed that the urban heat island effects have little influence on global mean temperature trends. Many studies reveal increases in the severity of the effect with the progress of climate change.〔Sachindra, D. A., Ng, A. W. M., Muthukumaran, S., & Perera, B. J. C. (2015). Impact of Climate Change on Urban Heat Island Effect and Extreme Temperatures: A Case Study. Quarterly Journal of the Royal Meteorological Society..〕 ==Causes== There are several causes of an urban heat island (UHI); for example, dark surfaces absorb significantly more solar radiation, which causes urban concentrations of roads and buildings to heat more than suburban and rural areas during the day;〔 materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This causes a change in the energy budget of the urban area, often leading to higher temperatures than surrounding rural areas. Another major reason is the lack of evapotranspiration (for example, through lack of vegetation) in urban areas. With a decreased amount of vegetation, cities also lose the shade and cooling effect of trees, and the removal of carbon dioxide.〔Santos, Fabiane. Trees – the Natural Air Conditioners. Scientific Scribbles. The University of Melbourne, 23 Aug. 2013. Web. 27 Sept. 2013. Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and prevents pollution from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI.〔 High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere.〔 As UHI raises the temperature of cities, it also increases the concentration of ozone, a greenhouse gas whose production accelerates with an increase in temperature.〔Union of Concerned Scientists. "Rising Temperatures, Worsening Ozone Pollution." Climate Change and Your Health (2011): n. pag. Print.〕 Some cities exhibit a heat island effect, largest at night. Seasonally, UHI shows up both in summer and winter. The typical temperature difference is several degrees between the center of the city and surrounding fields. The difference in temperature between an inner city and its surrounding suburbs is frequently mentioned in weather reports, as in " downtown, in the suburbs". "The annual mean air temperature of a city with 1 million people or more can be warmer than its surroundings. In the evening, the difference can be as high as ."〔United States. Environmental Protection Agency. Heat Island Effect. By EPA. N.p., 29 August 2013. Web. 31 August 2015. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「urban heat island」の詳細全文を読む スポンサード リンク
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