URBAN MICROCLIMATES - MORE RAIN AND WIND!

As well as being warmer, many cities are also damper.The increased heat can lead to more vigourous convectional activity over urban areas, and the rising thermals can modify, and warm up, any air passing over the city. These effects can therefore lead to enhanced precipitation.

The warmer air will in any case hold more water vapour, and coupled with the presence of dust and other hygroscopic pollutants, the vapour will condense around the abundant condensation nuclei, and produce more rain. The convectional cells will also lead to the risk of more thunderstorms in urban areas, and B.W.Atkinson (1968) found that thunder was heard overhead on 110 days in Central London between 1951 and 1960, compared with just 60 days on the Kent coast.

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Click here to see the results of Atkinson`s survey of thunder rain in London

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There are also important changes to the wind patterns as well, with air being blown over the irregular surface of an urban area. The taller buildings disturb the airflows and can create turbulence, with eddies and vortices as the air spirals in between and around buildings. These frictional effects can also cause dust and other rubbish to be swirled around, making walking along city streets very difficult and dangerous at times.

Winds can also become funneled down the city streets, in between the `canyons` created by rows of tall buildings and skyscrapers. This is known as the Venturi Effect, and it can result in faster wind speeds compared with rural areas.

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REFERENCES AND FURTHER READING