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Jul 21, 2014

An urban heat island, or UHI, is a metropolitan area that's a lot warmer than the rural areas surrounding it. Heat is created by energy from all the people, cars, buses, and trains in big cities like New York, Paris, and London. Urban heat islands are created in areas like these: places that have lots of activity and lots of people.

Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.

In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C). 

Characteristics of Urban Heat Island (UHI) :

As a result of urban development, urban areas have experienced a more rapid increase in temperature than the surrounding rural, agricultural, and wild land areas. Some general characteristics of the UHI are as follows.

  • Average minimum temperatures increase more than maximum temperatures.

  • Average nighttime temperatures increase more than the average daytime temperatures.

  • Urban areas experience year-round increases in the minimum daily temperatures.

  • The UHI causes longer daily warm periods and shorter daily cool periods.

  • Compared to rural areas, the duration of the hot season is extended in urban areas.

  • Temperatures can substantially vary between urban neighborhoods—the amount of vegetation plays a significant role in damping the 

Causes of UHI:

a) Anthropogenic heat release - Heat derived from industry and domestic central heating radiates from cities. 

b) The fabric of a city - Heat retaining (non-reflective) and emitting properties of building materials means that they absorb much of the incident radiation, which in turn is released as heat. This is called the albedo effect.

c) The lack of vegetation within towns and cities means that less light is intercepted, less evapotranspiration, thus less vapour is released (which naturally cools air surrounding vegetation). 

d) The close arrangement of high-rise buildings form urban street canyons found in most cities also contribute to the trapping of heat energy. The dynamics leading to this process is due to the reduced sky view factor that inhibits the escape of reflected radiation back into the atmosphere, causing the radiation to be absorbed by neighboring structures. 

Impact of Urban Heat Island  :

Elevated temperature from urban heat islands, particularly during the summer, can affect a community's environment and quality of life. While some heat island impacts seem positive, such as lengthening the plant-growing season, most impacts are negative and include:

  • Increased energy consumption: Higher temperatures in summer increase energy demand for cooling and add pressure to the electricity grid during peak periods of demand. One study estimates that the heat island effect is responsible for 5–10% of peak electricity demand for cooling buildings in cities.

  • Elevated emissions of air pollutants and greenhouse gases: Increasing energy demand generally results in greater emissions of air pollutants and greenhouse gas emissions from power plants. Higher air temperatures also promote the formation of ground-level ozone.

  • Compromised human health and comfort: Warmer days and nights, along with higher air pollution levels, can contribute to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.

  • Impaired water quality: Hot pavement and rooftop surfaces transfer their excess heat to storm water, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Rapid temperature changes can be stressful to aquatic ecosystems.

  • UHIs spoil water quality: Hot pavement and rooftop surfaces transfer their excess heat to storm water, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Additionally, increased urban water body temperatures lead to a decrease in diversity in the water



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