eTheses Repository - University of Birmingham

A single, distinct type of UHI does not exist, and it is possible to define many different types,
each with its own spatial and temporal characteristics (Oke 1982). Examples are the urban
boundary layer heat island (increased temperatures in the atmosphere above the city), the canopy
layer heat island (increased temperatures of the atmosphere between the ground and the mean
building height) and the surface heat island (this refers to the difference in surface temperature
between the urban and rural surface).
The intensity of the UHI is defined as the maximum difference in temperature between an urban
and rural location within a defined time period, for example a diurnal cycle. The intensity of the
canopy layer UHI depends strongly on the weather conditions and synoptic wind (e.g. Shea et al.
1978; Wong et al. 1978; Tumanov et al. 1999). It is highest during anti-cyclonic conditions, with
clear skies and light winds (Klysik et al. 1999; Pinho et al. 2000; Morris et al. 2001; Gedzelman
et al. 2003). Atmospheric fronts (whether warm, cold or occluded) act to enhance air mixing,
with the effect of equalling urban and rural temperatures and weakening the UHI intensity
(Tumanov et al. 1999). Strong winds will also significantly weaken or cancel out the UHI
(Morris et al. 2001).
A distinct diurnal and seasonal course has been documented for the UHI intensity (e.g. Klysik et
al. 1999; Gedzelman et al. 2003; Shepherd 2005). During daytime the UHI is less intense and can
even vanish, whereas during the night it reaches its greatest intensity. This is due to the release of
heat absorbed during the day by building materials. For many mid latitude cities the UHI
intensity is greatest in summer months. During winter months, although the anthropogenic heat
flux tends to be greater, there is also much less daytime solar energy to be absorbed and
subsequently released by the buildings. This tends to weaken the UHI intensity.
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