Geoscience Reference
In-Depth Information
in urban-rural temperatures is related statistically to
population size, being nearly linear with the logarithm
of the population. In North America, the maximum
urban-rural temperature difference reaches 2.5°C for
towns of 1000, 8°C for cities of 100,000 and 12°C
for cities of one million people. European cities show a
smaller temperature difference for equivalent popula-
tions, perhaps as a result of the generally lower building
height.
A convincing example of the relationship between
urban growth and climate is for Tokyo, which expanded
greatly after 1880 and particularly after 1946 (Figure
12.28A) The population increased to 10.4 million in
1953 and to 11.7 million in 1975. During the period
1880 to 1975, there was a significant increase in mean
January minimum temperatures and a decrease in the
number of days with minimum temperatures below 0°C
(Figures 12.28B and C). Although the graphs suggest
a reversal of these trends during the Second World War
(1942 to 1945), when evacuation almost halved Tokyo's
population, it is clear that the basis of correlations
of urban climate with population is complex. Urban
density, industrial activity and the production of anthro-
pogenic 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 to 300 m,
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 to 3°C in summer. Low solar elevation
angle causes shading of urban streets, in contrast to
locations outside the built-up area.
Figure 12.28
The built-up area of Tokyo in 1946 (A), the mean
January minimum temperature (B), and number of days with
subzero temperatures (C) between 1880 and 1975. During the
Second World War, the population of the city fell from 10.36
million to 3.49 million and then increased to 10.4 million in 1953
and to 11.7 million in 1975.
Source
: After Maejima
et al
. (1982).
3 Modification of surface characteristics
a Airflow
Although it is difficult to isolate changes in temper-
atures that are due to urban effects from those due to
other climatic factors (see Chapter 13), it has been
suggested that city growth is often accompanied by
an increase in mean annual temperature. At Osaka,
Japan, temperatures have risen by 2.6°C in the past 100
years. Under calm conditions, the maximum difference
On average, city wind speeds are lower than those
recorded in the surrounding open country owing to the
sheltering effect of the buildings. Average city-centre
wind speeds are usually at least 5 per cent less than
those of the suburbs. However, the urban effect on air
motion varies greatly depending on the time of day
and the season. During the day, city wind speeds are
