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Is the weather different around urban cities, or centres? If so why?

Question submitted by Curtis Bennett
(Kelowna, BC)

This is a great question Curtis, and the answer is "absolutely"! The effect is known as the Urban Heat Island (UHI) effect.

An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surrounding rural areas.
http://i.treehugger.com/files/th_images/urban_heat_island.jpg
The observed temperature difference is larger at night than during the day, and is most apparent when winds are weak. Seasonally, UHI is seen during both summer and winter. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat; waste heat generated by energy usage is a secondary contributor. As population centers grow they tend to modify a greater and greater area of land and have a corresponding increase in average temperature.

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 is even thought to potentially decrease the occurrence of weak tornadoes (more research is needed for thorough confirmation). Increases in the death rate during heat waves has been shown to increase by latitude due to the urban heat island effect. 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, which stresses their ecosystems.

Not all cities have a distinct urban heat island, however. 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, any impact of the urban heat island on global warming is uncertain, its impact on climate change has not been proved observationally or by any quantitative modelling, though recent qualitative speculations indicate that urban thermal plumes may contribute to variation in wind patterns that may itself influence the melting of arctic ice packs and thereby the cycle of ocean current.

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