The temperature variations existent in urban heat islands are even more significant in the winter, due to the greater addition of domestic heating systems to the atmosphere. In general, there are significant increases in temperature occurring increasing from the outer areas to the centre by about 2 – 4 C°. The specific pattern of temperature variation
b) Examine the factors influencing the spatial and temporal variations in the micro-climates of urban areas. (20)
Urban microclimates refer to the differing local climatic conditions of a built up area, which tend to vary significantly from those of surrounding areas. The built-up area’s microclimate tends to display distinctive features of temperature, wind, visibility and precipitation, which may vary spatially across particularly large urban areas. The impact of may extend up to 300m upwards and as much as 10km downwind. Some geographers assert that there are two zones to an urban heat island which can be categorised: the urban canopy – the area below the average roof level, where process act within the gaps between the buildings. And secondly the urban boundary layer which is found above, whose characteristics are governed by the nature of the urban surface
The specific pattern of temperature variation will be dictated by the albedo of the surfaces found. Grass covered areas within the city or town will reflect much more insolation than concrete and tarmac surfaces, consequently temperatures recorded here will be relatively lower than other areas within the microclimate. 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.
Previously visibility was a major issue affecting urban microclimates. In industrial cities, smog was a very common hindrance to its people. The smog was created by excessive combustion of fossil fuels, which produced harmful emission that helped to reduce visibility, this coupled with foggy weather conditions led to the creation of a rather unpleasant unique weather system found exclusively in urban microclimates. However most developed countries have now introduced legislation that has eradicated the problem.
Finally higher precipitation levels usually exist in urban areas in contrast to adjacent rural areas as a consequence of: the abundance of tall buildings providing a micro scale orograghic effect, the very active convection taking place due to the urban heat island effect, the higher concentration of particulate pollutants which help to provide condensation nuclei and also the fact that power stations and industrial sources located in urban areas emit water vapour as steam. All of these conditions help to explain the higher than average rainfall that takes place in urban areas, so much so that there is 10% more rainfall on average in urban areas in opposition to neighbouring rural areas. Other variations in the urban micro climate include: the lack of water and vegetation which helps to reduce the level of evapotranspiration and the amount of humidity – particularly during the summer period. The higher temperatures also mean that snowfall is much less common than in rural areas, and even if it does occur, it thaws relatively quickly. As a result snow days are almost 15% less likely in urban areas than in rural ones. 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.
