Geoscience Reference
In-Depth Information
Figure 12.29
Details of urban air-
flow 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.
Notes
: SP = stagnation point; CS =
corner stream; VF = vortex flow; L =
lee eddy.
Sources
: After Plate (1972) and Oke
(1978).
considerably lower than those of surrounding rural
areas, but during the night the greater mechanical
turbulence over the city means that the higher wind
speeds aloft are transferred to the air at lower levels by
turbulent mixing. During the day (13:00 hours), the
mean annual wind speed for the period 1961 to 1962 at
Heathrow Airport (open country within the suburbs)
was 2.9 m s
-1
, compared with 2.1 m s
-1
in central
London. The comparable figures at night (01:00 hours)
were 2.2 m s
-1
and 2.5 m s
-1
. Rural-urban wind speed
differences are most marked with strong winds, and the
effects are therefore more evident during winter when
a higher proportion of strong winds is recorded in mid-
latitudes.
Urban structures affect the movement of air both by
producing turbulence as a result of their surface rough-
ness and by the channelling effects of the urban canyons.
Figure 12.29 gives some idea of the complexity of
airflow around urban structures, illustrating the great
differences in ground-level wind velocity and direction,
the development of vortices and lee eddies, and the
reverse flows that may occur. Structures play a major
role in the diffusion of pollution within the urban
canopy; for example, narrow streets often cannot be
flushed by vortices. The formation of high-velocity
streams and eddies in the usually dry and dusty
urban atmosphere, where there is an ample debris
supply, leads to general urban airflows of only 5 m s
-1
being annoying, and those of more than 20 m s
-1
being
dangerous.
drains reduces local evaporation. The lack of extensive
vegetation cover eliminates much evapotranspiration,
and this is an important source of augmenting urban
heat. For these reasons, the air of mid-latitude cities has
a tendency towards lower absolute humidity than that of
their surroundings, especially under conditions of light
winds and cloudy skies. During calm, clear weather, the
streets trap warm air, which retains its moisture because
less dew is deposited on the warm surfaces of the city.
Humidity contrasts between urban and rural areas are
most noticeable in the case of relative humidity, which
can be as much as 30 per cent less in the city by night as
a result of the higher temperatures.
Urban influences on precipitation (excluding fog) are
much more difficult to quantify, partly because there
are few rain gauges in cities and partly because turbulent
flow makes their 'catch' unreliable. It is fairly certain,
however, that urban areas in Europe and North America
are responsible for local conditions that, in summer
especially, can trigger off excesses of precipitation
under marginal conditions. Such triggering involves
both thermal effects and the increased frictional conver-
gence of built-up areas. European and North American
cities tend to record 6 to 7 per cent more days with rain
per year than their surrounding regions, giving a 5 to 10
per cent increase in urban precipitation. Over southeast
England between 1951 and 1960, summer thunderstorm
rain (which comprised 5 to 15 per cent of the total
precipitation) was especially concentrated in west,
central and south London, and contrasted strikingly with
the distribution of mean annual total rainfall. During
this period, London's thunderstorm rain was 20 to
25 cm greater than that in rural southeast England. The
effect is generally more marked in the cold season in
North America, although urban areas in the Midwest
b Moisture
The absence of large bodies of standing water in urban
areas and the rapid removal of surface runoff through
