To what extent do urban areas modify their climates? (40 marks)

Urban areas climate is often significantly different to the surrounding rural areas, this is why urban areas are often described as having their own “micro climate” the differences in urban climates are due to number of different factors.
Urban areas often experience a phenomenon known as a heat island, this is a zone of hot air around and above an urban area which has higher temperatures than the surrounding rural areas consequently cities tend to be warmer than rural areas and the temperature becomes lower progressively as you move towards the rural area with the highest temperatures being in the city centre. Heat islands form due to a variety of factors which are present within cities and not commonly found in rural areas, firstly building material such as concrete and tarmac absorb large quantities of heat during the day, due to their dark colours, and when temperatures are cooler, at night, the heat is released gradually warming up the surrounding area, this is why the effects of the heat island are more visible at night time because the city remains warm whilst the rural areas are much cooler. There are a large amount of buildings with glass windows within the urban areas, these have a high reflective capacity and reflect heat towards the streets were it is absorbed. Another factor influencing towards to the heat island formation is the heat energy being emitted from industry due to the combustion of fossil fuel which releases heat, house heating also contributes towards this, this heat energy warms up the air within the urban area contributing to the dome of warm air which is the heat island. The emission of pollutants from cars also acts as a particle in their air, forming condensation and therefore smog.
Urban heat islands can also have an effect on pressure. Locally, as warm air rises over an urban area it draws in heat from the surrounding area and creates an area of localised low pressure. Pressure is also another factor in influencing the climate of an urban area. Strong pressure gradients develop between the windward and leeward side of buildings and can lead to severe eddying winds. Pressure gradient is the main driving force that causes the movement of wind from areas of high pressure to areas of low pressure. High pressure conditions create calm country breezes which move from colder rural areas to warmer urban area. This is due to the fact that as warm air rises it creates a centre of low pressure below, air blows from high pressure to low pressure, this means winds converge on central areas and consequently may bring pollution from outer areas, into the city centre. There are often patterns and differences between rural and urban areas and these comparisons are greatest seen under calm high pressure conditions. It is clear urban winds are different from rural ones in terms of their speed and direction.
Winds in an urban microclimates are affected by it surroundings. The general pattern is that winds decrease in urban areas than in suburbs or rural areas. The speed of winds is lower in altitudes in built up areas are slower because winds are deflected over high buildings. Another factor affecting urban climates is the buildings, which affect the winds due to their size, shape and spacing. Tall buildings provide frictional drag on the airs movement. The frictional drag creates turbulence, which gives rapid changes in the direction and speeds of the wind. Pressure gradients are set up between the windward and leeward side of buildings which causes the eddying of the winds. The windward side of a building tends to have high pressure due to the air pushing against it, the leeward side which blocks the straight path of the wind has an area of low pressure and the wind moves around the building to the area of low pressure causing a steep pressure gradient. The steep pressure gradient causes these winds to be strong. When high pressure conditions occur in rural areas breezes move towards the low pressure created in the urban area by the rising convectional heat currents, the air therefore moves from the high pressure in the rural areas to low pressure in the city. The spacing of buildings can also affect winds. Single buildings which are widely spaced apart, act on the wind by themselves. Closely spaced buildings work together with each other creating more friction drag causing winds to skim over the top and eddy in between them. Buildings also act as wind channels where the wind moves at faster speeds causing pedestrians to be blown over and causing a lot of litter. Chicago is nick named the windy city due to its grid system of buildings creating wind tunnels where the wind can pick up speed. However, some areas will have less of an effect of the wind speed and direction because they may not be laid out in a grid and therefore not channel wind as much.
To conclude, it is clear that urban areas do affect their climate, their micro climate, but not the world’s macro climate. It would take a large increase in urban are size to affect the macro climate. The micro climate, like heat and wind are definitely affected.