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Geography - Atmospheric Systems Essay a) Outline the characteristics of urban heat islands. (5) First and foremost the urban heat island can be defined as an effect whereby inner city areas tend to have higher mean annual and also higher winter minimum temperatures than the surrounding rural areas. The differences in heat usually become more substantial towards the centre of the urban areas. Heat is given off by factories, vehicles and home, all of which burn fuel and produce heat which aids the urban heat island effect. Also urban surfaces, such as concrete and tarmac, absorb substantial amounts of solar radiation before releasing it during the night. This occurrence therefore attributes to the much higher night time temperatures found in the centres of urban areas. Smog and pollutants found in the urban areas form a pollution zone which allows short wave insolation to enter, however the smog consequently traps the outgoing terrestrial radiation as this is of a longer wavelength.
The release of heat from buildings is slow; consequently changes in urban temperatures often lag behind seasonal patterns. The wind is another key factor which has an influence on the extent of the urban microclimate. Usually built up areas tend to have lower wind speeds than surrounding areas, this is because taller buildings provide a frictional drag on air movements. Consequently, wind turbulence is created, providing rapid changes in both its direction and speed. Therefore a general decrease in wind speed occurs as the air travels to the city centre from the suburbs. In rare cases, the urban heat island effect may actually alter the local wind patterns completely. Indeed a low pressure area may develop as warm air rises over the urban area. As a result winds in urban areas tend to be 20-30% lower than average. However planners must consider the importance of a well designed urban area, as efficient air flow is essential so that damaging pollutants can be dispersed.
On the other hand, thunderstorms are much more common in built up areas, due to the intense convection that can occur, particularly during hot summer evenings. An interesting example of an influential factor that specifically affects an urban heat island is the urban rainfall in Manchester - a city in northwest England. Recent research undertaken by a local university has suggested that the erection of a band of high rise tower blocks in Manchester during the 1970s has brought more rain to certain parts of the city. The level of rainfall has increased by approximately 7% over recent decades. This has occurred as a consequence of the turbulence made by the micro scale effect of tall buildings, which forces the air to rise. Also the heat island effect means that the temperature can be up to 8°C higher in the centre of the city in comparison to the surrounding countryside. This contributes further to the above average rainfall, by causing the air to rise further as a result of convection. ?? ?? ?? ?? Alex Potter 6N2 23rd March 2009
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