Urban heat island
Urban areas usually experience the urban heat island (UHI) effect, that is, they are significantly warmer than surrounding rural areas. The temperature difference is usually larger at night than during the day,[1] and is most apparent when winds are weak, under block conditions, noticeably during the summer and winter. The main cause of the UHI effect is from the modification of land surfaces while waste heat generated by energy usage is a secondary contributor.[2][3][4] A study has shown that heat islands can be affected by proximity to different types of land cover, so that proximity to barren land causes urban land to become hotter and proximity to vegetation makes it cooler.[5] As a population center grows, it tends to expand its area and increase its average temperature. The term heat island is also used; the term can be used to refer to any area that is relatively hotter than the surrounding, but generally refers to human-disturbed areas.[6] Urban areas occupy about 0.5% of the Earth's land surface but host more than half of the world's population.[7]
This article is about higher temperatures in cities due to urbanization effects. For effects of climate change on city temperatures, see climate change and cities.
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, and the heat island characteristics depend strongly on the background climate of the area in which the city is located.[8] Effects within a city can vary significantly depending on local environmental conditions. Heat can be reduced by tree cover and green space, which act as sources of shade and promote evaporative cooling.[9]
Other options include green roofs, passive daytime radiative cooling applications, and the use of lighter-colored surfaces and less absorptive building materials in urban areas, to reflect more sunlight and absorb less heat.[10][11][12]
Climate change is not the cause of urban heat islands but it is causing more frequent and more intense heat waves which in turn amplify the urban heat island effect in cities.[13]: 993 Compact, dense urban development may increase the urban heat island effect, leading to higher temperatures and increased exposure.[14]
Society and culture[edit]
History of research[edit]
The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon.[88] A description of the very first report of the UHI by Luke Howard said that the urban center of London was warmer at night than the surrounding countryside by 2.1 °C (3.7 °F).[89]
Investigations of the urban atmosphere continued throughout the nineteenth century. Between the 1920s and the 1940s, researchers in the emerging field of local climatology or microscale meteorology in Europe, Mexico, India, Japan, and the United States pursued new methods to understand the phenomenon.
In 1929, Albert Peppler used the term in a German publication believed to be the first instance of an equivalent to urban heat island: städtische Wärmeinsel (which is urban heat island in German).[90] Between 1990 and 2000, about 30 studies were published annually; by 2010, that number had increased to 100, and by 2015, it was more than 300.[91]
Leonard O. Myrup published the first comprehensive numerical treatment to predict the effects of the urban heat island (UHI) in 1969.[28] His paper surveys UHI and criticizes then-existing theories as being excessively qualitative.