However this does not only impact the wind speed and direction, it also affects pollution in urban areas. As there is little disturbance between buildings, the air tends to stagnate more easily which means the pollutants are not flushed out well. Similarly the urban ‘canyons’ in urban areas, created by winds being squeezed into restricted streets, produces the venturi effect. This removes the pollution from the urban area and moves it to other areas. For example, this happens in East of London. However other aspects of the urban areas affect the impact of pollution. The photochemical smog has had increasing impact due to coal burning. Together with strong sunlight and increased car use, this has created photochemical smog which impacts climatic characteristics. For example, in Winter there has been 100% increase in fog in urban areas, compared to rural areas. The significance of the impact of pollution can be seen in the various measures being taken to reduce its effect in urban areas. For example, in 1990s tough regulations were imposed on levels of airbourne pollution, particularly on the levels of PM10s in the atmosphere. Overall this urban pollution and smog can trap outgoing radiant energy, showing the significant impact on the temperature - a climatic characteristic.
Another impact that urban areas have is on the humidity. Overall relative humidity is lower in cities than surrounding area, showing a significant impact. It is 2% less in winter and 8% less in summer due to the rapid disposal of water, meaning less surface water. This is because there are fewer areas of water and less vegetation, reducing rates of evapotranspiration. However at night, the urban area maintains its humidity whereas the air cools more quickly in rural areas and moisture is lost to dew. Overall the impact of urban areas is significant, as seen by the figures. The main reason for this is the constant attempt to reduce surface water, for example by drains, in urban areas.
However this presence of water in urban areas has a significant impact on the temperature of urban areas also. Any left over latent heat, not used during evapotranspiration, is converted into sensible heat - the heat that is felt and therefore raises the temperature in urban areas. There is less evapotranspiration in urban areas due to the lack of surface water and vegetation, causing a rise in this climatic characteristic. The presence of pollutants also raises temperature as the pollutant dome, present in urban areas, can absorb heat and prevent some escaping. This pollutant dome will re-radiate heat down to the urban surface. Similarly the height and arrangement of buildings will also impact the temperature. It is true that the vertical aspect of tall buildings close together will generate heat and therefore urban areas, where these tall buildings are a common characteristic, generate more heat than rural areas. Clearly this will impact on this climatic characteristic. In addition, the tall buildings will reduce the sky view factor which means large amounts of sunlight and therefore heat is lost to atmosphere due to interception. Another factor causing the higher temperature is the type of building materials. For example, the heat capacity of concrete is 1/3 that of wet mud so concrete will warm up more rapidly.
Therefore this suggests that urban areas have a significant impact on the temperature. This creates an ‘urban heat island’, as it is 1-2℃ warmer per year than surrounding areas. The fact that this warm spot in a sea of cooler rural temperatures exists suggests that urban areas have a significant impact on temperature. This is particularly due to the high buildings which have a higher capacity to retain and conduct heat. Similarly they have a lower albedo, meaning the heat is not reflected easily. This is a good example of the impact of human activity on urban climate. The impact of this activity is so great that it affects several climatic characteristics. However the significance of these depends on where the urban area is located, for example. One example of this is that cities in North America have greater temperature difference than cities in Europe. This is due to the higher number of high density buildings in Europe. Similarly, there is greater city planning in North America. This shows that, although urban areas have an impact on climatic characteristics, it may depend on the location. Another factor that may affect the significance of the impact is the amount of anthropogenic heat. These activities will inject heat into the canopy layer and upper boundary layer, however the amount of activity may depend on the urban area. For example, some areas are now trying to encourage the use of public transport, suggesting a decline in heat released from vehicles and therefore anthropogenic heat. In conclusion, it is clear that Urban Areas have a significant impact on climatic characteristics, particularly on the temperature. However this temperature and urban heat island is a result of several factors and the greatness of these factors will affect the significance of the impact.
