eTheses Repository - University of Birmingham

For mid-latitude cities, of which London is an example, the UHI typically displays a
market seasonal and diurnal pattern, with the strongest UHI intensities recorded on summer
nights (due to the release of heat absorbed by buildings during the day), and generally
weaker intensities during the winter (despite greater anthropogenic heat flux) due to the
reduction in the absorption (and subsequent release) of solar energy. Graves et al. (2001)
found that the London UHI is most pronounced at night time (with a near zero intensity
observed for the mid afternoon period in summer months), and in calm, clear conditions,
that the intensity weakens with increasing wind speed and distance from the centre, and
that the location of the maximum UHI intensity shifts with changes in wind direction
(typically by several kilometres). The UHI is also highly dependent on weather patterns,
and it can vary widely from one day to the next. Wilby (2003) found that the nocturnal UHI
was strongest in the summer (peaking at 2.2 °C in August) and weakest in the winter (1.1
°C in January).
Graves et al. (2001) did not observe a weekly cycle in the UHI intensity in summer months.
This suggests that there is no weekly cycle in the heat released from buildings and traffic.
In winter months it is expected that the anthropogenic heat flux will play a more significant
role, as an increase in UHI intensity from late afternoon to mid morning is observed
(Threlfall 2001).
2.4.4 London Warming? Trends in the UHI intensity
Lee (1992) analysed recent trends in London’s heat island over a period characterised by a
decline in population due to counter-urbanisation. It is difficult to establish whether the
observed trends are due to the change in population, or other factors, such as variations in
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