Geoscience Reference
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is complex. Urban density, industrial activity and
the production of anthropogenic heat are all
involved. Leicester, England, for example, when it
had a population of 270,000, exhibited warming
comparable in intensity with that of central
London over smaller sectors. This suggests that the
thermal influence of city size is not as important
as that of urban density. The vertical extent of the
heat island is little known, but is thought to exceed
100-300m, especially early in the night. In the
case of cities with skyscrapers, the vertical and
horizontal patterns of wind and temperature are
very complex (see
Figure 12.29
).
In some high-latitude cities there is a reverse
'cold island' effect of 1-3°C in summer. In the
United States this effect has been reported in
Boston, MA, Dallas, TX, Detroit, MI and Seattle,
WA, when corrections are made to temperature
for latitude and elevation differences. At the
microscale, low solar elevation angle causes
shading of urban streets, in contrast to locations
outside the built-up area. A similar cool island is
observed in cities in tropical and subtropical
deserts where it is attributed to the high thermal
inertia of the built-up area, and the sharp diurnal
temperature fluctuations. Its intensity depends on
the orientation of the street canyons - increasing
as street-axis orientation approaches north-south
and decreasing to near zero in the east-west
direction (see below). One recent study, where
inhomogeneities and biases in the temperature
data were carefully examined, finds no statistically
significant impact of urbanization in the
contiguous United States; as many cities showed
a cold island as a heat island. A suggested reason
for this is that micro- and local-scale impacts
(A)
Built-up area
1946
1975
Bay of Tokyo
0
10
km
(B)
2
0
-2
-4
-6
100
(C)
80
60
SP
40
L
20
0
1.3
CS
1880
1900
1920
1940
1960
1980
3.0
2.5
VF
Figure 12.28
A: The built-up area of Tokyo in
1946. B: The mean January minimum temperature.
C: Number of days with sub-zero temperatures
between 1880 and 1975. During World War II, the
population of the city fell from 10.36 million to 3.49
million and then increased to 10.4 million in 1953
and 11.7 million in 1975.
Source: “After”, Maejima et al. (1980). Courtesy of Professor
J. Matsumoto. Courtesy of Professor J. Matsumoto.
1.0
Figure 12.29
Details of urban airflow around two buildings
of differing size and shape. Numbers give relative wind
speeds; stippled areas are those of high wind velocity and
turbulence at street level.
Sources: After Plate (1972) and Oke (1978).
Notes: SP = stagnation point; CS = corner stream; VF = vortex flow;
L = lee eddy.
