Geoscience Reference
In-Depth Information
properties of the surface. It is thought that the canyon
geometry effect dominates in the urban canopy layer,
whereas the sensible heat input from urban surfaces
determines the boundary layer heating. By day, the
urban boundary layer is heated by increased absorption
of short-wave radiation due to the pollution, as well as
by sensible heat transferred from below and entrained
by turbulence from above.
The
heat island
effect may result in minimum urban
temperatures being 5 to 6°C greater than those of the
surrounding countryside. These differences may reach
6 to 8°C in the early hours of calm, clear nights in large
cities, when the heat stored by urban surfaces during the
day (augmented by combustion heating) is released.
Because this is a
relative
phenomenon, the heat island
effect also depends on the rate of rural cooling, which
is influenced by the magnitude of the regional envi-
ronmental lapse rate.
For the period 1931 to 1960, the centre of London
had a mean annual temperature of 11.0°C, compared
with 10.3°C for the suburbs, and 9.6°C for the surround-
ing countryside. Calculations for London in the 1950s
indicated that domestic fuel consumption gave rise to a
0.6°C warming in the city in winter and this accounted
for one-third to one-half of the average city heat excess
compared with adjacent rural areas. Differences are
most evident during still air conditions, especially
at night under a regional inversion (Figure 12.27).
For the heat island effect to operate effectively there
must be wind speeds of less than 5 to 6 m s
-1
. It is
especially apparent on calm nights during summer and
early autumn, when it has steep cliff-like margins on
the upwind edge of the city and the highest tempera-
tures are associated with the highest density of urban
dwellings. In the absence of regional winds, a well-
developed heat island may generate its own inward
local wind circulation at the surface. Thus the thermal
contrasts of a city, like many of its climatic features,
depend on its topographic situation and are greatest for
sheltered sites with light winds. The fact that urban-
rural temperature differences are greatest for London in
summer, when direct heat combustion and atmospheric
pollution are at a minimum, indicates that heat loss from
buildings by radiation is the most important single factor
contributing to the heat island effect. Seasonal differ-
ences are not necessarily the same, however, in other
macroclimatic zones.
The effects on minimum temperatures are especially
marked. For central Moscow, winter extremes below
-28°C occurred only eleven times during 1950 to 1989
compared with twenty-three cases at Nemchinovka west
of the city. Cologne, Germany, has an average of 34
per cent fewer days with minima below 0°C than its
surrounding area. In London, Kew has an average of
seventy-two more days with frost-free screen temper-
atures than rural Wisley. Precipitation characteristics
are also affected; incidences of rural snowfall are often
associated with either sleet or rain in the city centre.
Figure 12.27
Distribution of minimum tem-
peratures (°C) in London on 14 May 1959,
showing the relationship between the 'urban
heat island' and the built-up area.
Source
: After Chandler (1965).
