The urban heat island (UHI) effect is a meteorological and climatological phenomenon in which urban areas experience significantly warmer temperatures than surrounding rural areas. The temperature difference is usually larger at night than during the day, 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. Urban areas occupy about 0.5% of the Earth's land surface but host more than half of the world's population. 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. Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increase the length of growing seasons, decrease air quality by increasing the production of pollutants such as ozone, and decrease 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 where the city is located. The impact in a city can significantly change based on its local environment. Heat can be reduced by tree cover and green space, which act as sources of shade and promote evaporative cooling. Other options include green roofs, passive daytime radiative cooling applications, ventilation corridors, the use of lighter-colored surfaces, and less absorptive building materials. These reflect more sunlight and absorb less heat. 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. Compact and dense urban development may also increase the urban heat island effect, leading to higher temperatures and increased exposure.
Definition A definition of urban heat island is: "The relative warmth of a city compared with surrounding rural areas." This relative warmth is caused by "heat trapping due to land use, the configuration and design of the built environment, including street layout and building size, the heat-absorbing properties of urban building materials, reduced ventilation, reduced greenery and water features, and domestic and industrial heat emissions generated directly from human activities".
Description
Diurnal variability
Throughout the daytime, particularly when the skies are cloudless, urban surfaces are warmed by the absorption of solar radiation. Surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. By virtue of their high heat capacities, urban surfaces act as a reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. "This study shows that urban surfaces such as concrete absorb and store large amounts of heat during the day, confirming the high heat capacity effect described in urban heat islands." As a result, high daytime surface temperatures within the UHI can be easily seen via thermal remote sensing. As is often the case with daytime heating, this warming also has the effect of generating convective winds within the urban boundary layer. At night, the situation reverses. The absence of solar heating leads to the decrease of atmospheric convection and the stabilization of urban boundary layer. If enough stabilization occurs, an inversion layer is formed. This traps urban air near the surface, keeping surface air warm from the still-warm urban surfaces, resulting in warmer nighttime air temperatures within the UHI. Generally speaking, the difference in temperature between the urban and surrounding rural area is more pronounced at night than in daytime. For example, in the United States, the temperature in urban areas tends to be warmer than the surrounding area by about 1–7 °F (0.55–3.9 °C) during the daytime, and about 2–5 °F (1.1–2.8 °C) warmer at night. However, the difference is more pronounced during the day in arid climates such as those in southeastern China and Taiwan. Studies have shown that diurnal variability is impacted by several factors including local climate and weather, seasonality, humidity, vegetation, surfaces, and materials in the built environment.
Seasonal variability Seasonal variability is less well understood than diurnal variability of the urban heat island temperature difference. Complex relationships between precipitation, vegetation, solar radiation, and surface materials in various local climate zones play interlocking roles that influence seasonal patterns of temperature variation in a particular urban heat island.
Measurements and predictions
Urban Heat Island Index (UHII) One method to quantify the UHI effect within urban areas is the UHI Index created by the Californian EPA in 2015. It compares the temperature of a surveyed area and rural reference points upwind from the surveyed area, at a height of two meters above ground level. The difference in temperature in degrees Celsius is taken hourly and differences with an increased urban temperature compared to the reference points are summed up, creating a number of degree-Celsius-hours, which is the UHI Index of the surveyed area. The measure of Celsius-hours might be averaged over many days, but is specified as Celsius-hours per averaged day. The index was created to estimate the expected use of air conditioning and resulting greenhouse gas emissions in California. The index does not consider values of or differences in wind-speed, humidity, or solar influx, which might influence perceived temperature or the operation of air conditioners.
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