eTheses Repository - University of Birmingham

Cold islands can also occur in urban areas, particularly during daytime when the UHI is less
intense and can become negative. This is due to increased energy being partitioned into storage,
which can be especially high due to the urban building vertical surfaces and heat capacities of the
construction materials.
For many cities in the world an increasing trend in the UHI intensity has been identified (e.g. Lee
1992; Velazquez-Lozada et al. 2006; He et al. 2007) and this is expected to be due to an increase
in population and the city area. However, it is not easy (or indeed possible) to establish a definite,
linear relationship between the population (as a surrogate indicator of city size) and the UHI
intensity. Although population was originally linked to the UHI development and its intensity,
Oke (1987) found that any relationship would differ in North American and European cities.
Also, cities located at tropical latitudes do not appear to fit into either range, probably due to
different urban-rural contrasts in soil moisture. The geographic location of a city (and the
corresponding regional climate, characteristics of the rural surroundings and the influences of
local topography) is clearly a controlling factor in determining the UHI intensity (e.g. Nitis et al.
2005). It can also be seen that characteristics such as city structure, population density, building
compactness, sky view factor, the percentage of artificial surfaces and vegetation fraction in the
city are important factors in controlling the UHI development (Oke 1987). Other factors such as
the frequency of suitable synoptic conditions and regional climatic fluctuations might also affect
long term trends (Chandler 1965). Nonetheless population growth could be an important factor in
the UHI development, since it is accompanied by increases in urban surface area, housing, roads,
public transport and other services, all of which affect the surface energy balance. In recent years
some major cities (for example London) have seen a decrease in population in the centre, with a
migratory flux towards the suburbs. However the decrease in population is not necessarily
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