- Building layout – increased surface area and multiple reflection
- Air pollution – greater absorption and re-emission
- Building layout – reduction of sky-view factor
- Building and traffic heat losses
- Construction materials – increased heat-storage capacity
- Construction materials – increased ‘waterproofing’; faster urban drainage
- Building layout – reduction of wind speed.
It is the change in the urban heat balance that leads to the rise in average annual mean temperatures. There are three main sources of energy into the urban environment solar radiation, heat from the earth’s core and anthropogenic heat. These inputs would be broadly similar for a rural environment expect for the anthropogenic heat which a t 15-50 Wm-2 for a typical mid-latitude city is not thought sufficient enough to create or sustain heat islands (Shahgedanova and Burt, 1998).
However the loss of energy through evaporation, conduction and radiation is significantly reduced by the urban built environment (Foskett and Foskett, 1992). Due to the urban environment being designed to remove water quickly less water in available for evaporation than compared with rural areas therefore less latent heat can be removed from the city. This therefore increases the amount of sensible heat within the urban system and this leads to a rise in air temperatures.
These factors affect a range of climatic elements within the urban heat island which contribute to the spatial distribution of temperature (Goudie, 1993). Radiation is reduced due to multiple reflections and absorption by the materials of the built environment. Wind speeds are reduced overall within a city though localised channelling can occur due to building lay out and this limits heat loss from the city. It is this reduction in the ability of an urban area to dissipate heat that causes urban heat islands.
Variations in relative humidity across the London area
Figure 3 shows the highly complex distribution of relative humidity across London and there seems to be a clear trend of reduced humidity towards the centre. However the cross section shows a distinct rise in humidity over the river Thames and this large body of water would increases evaporation and therefore humidity in this linear corridor. The Thames also passes through the hottest part of the heat island and this would also increase evaporation.
Urban heat island are a mosaic of microclimates and the high variation displayed in figure 3 is likely to be due to very localised land use for example an area of vegetation like a park which would have localised evaporation.
Overall humidity will be lower in the built environment as there are fewer water bodies, a lower rate of evapotranspiration and more rapid runoff of water (Foskett and Foskett, 1992). Also urban areas have greater humidity at night and lower humidity during the day compared with surrounding rural areas.
Variations in fog frequencies across the London area
Fog is a cloud in contact with the ground. It occurs when moisture from the surface of the Earth evaporates; as this evaporated moisture moves upward, it cools and condenses into the familiar phenomenon of fog. Fog differs from cloud only in that fog touches the surface of the Earth, while clouds do not. It can form in a number of ways, depending on how the cooling that caused the condensation occurred (Barry, and Chorley, 1998).
There are several types of fog which are classified by the different processes of formation though urban heat islands impact mostly on the formation of radiation fog. Radiation fog results from cooling of the earth’s surface after sunset and can cause water vapour to condense at the surface level (Ahrens, 1999).
As we have already seen the urbanisation results in a number of changes that affect the conditions required for fog formation namely quantity of nuclei for condensation, temperature, humidity and wind. Pollution in urban areas causes a greater availability of nuclei for fog formation. Temperature is increased within the urban heat island which could increase evaporation/water vapour if there was an available body of water but as we have seen urban areas have lower availability of water for evaporation. Finally overall wind speeds are reduced within urban areas due to the increase friction caused by building.
Kingsway in central London has 959 hours of fog per year but the vast majority, 940, is less dense fog. Kingsway would have high levels of pollution and temperature and low wind levels which would be ideal for fog formation as the ground surface slowly cools on clear nights. However figure 3 shows that Kingsway one of the lowest relative humidity levels in London and therefore this low level of water vapour limits the extent of fog formation though it is occurring regularly.
Kew has the highest level of dense fog and the second highest overall fog hours. Kew has a relative high level of vegetation and therefore higher levels of humidity and water vapour, it is also within a fairly high isotherm. However it would be slightly windier, slightly less pollution and therefore condensation nuclei. These conditions therefore reduce the frequency of conditions for fog formation but when conditions are suitable dense fogs are likely to form.
London Airport is at the boundary of the urban heat island and therefore temperature will be similar to the surrounding rural area also the prevailing wind is uninhibited by the built environment and therefore it will be windier. Humidity will be higher due to increase proportion of vegetation and there will be significant localised pollution from the airport. These conditions will counteract each other to a certain extent and therefore fog occurrence is only slightly greater than South-East England though is more likely to be dense due to the additional condensation nuclei originating from the airport.
Finally the mean of the 7 stations of South-East England should provide a representative frequency of fog occurrence outside of the urban heat island. This can be used as a reference to compare the three sites within the urban heat island. If you calculate the frequency of fog compared with the intensity the site at Kew has the most fog followed by London Airport, Kingsway and finally South-East England. Kew tops this calculation because it has an idea mixture of condensation nuclei (pollution), temperature, humidity and calm wind conditions.
Bibliography:
Ahrens, C.D. (1999) Meteorology Today: An Introduction to Weather, Climate, and the Environment. 6th edn. California: Brooks/Cole
Barry, G.B. and Chorley, R.J. (1998) Atmosphere, Weather and Climate. 7th edn. London: Routledge
Foskett, N and Foskett, R (1992) ‘Urban Climates’. Geofile January 1992, No.185
Goudie, A (1993) The Nature of the Environment.3rd eds. Oxford: Blackwell
Moran and Morgan (1997) Meteorology – The atmosphere and the science of weather. 5th eds. New Jersey: Prentice-Hall
Shahgedanova, M and Burt, T (1998) ‘Urban Heat Islands’. Geography Review Vol.11, No.3, p.36-41
