An urban heat island is a 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. 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, which use materials that effectively store short-wave radiation. Waste heat generated by energy usage is a secondary
contributor. As a population centre 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.
Despite concerns raised about its possible contribution to
global warming, comparisons between urban and rural areas show that the urban
heat island effects have little influence on global mean temperature trends.
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