Geoscience Reference
In-Depth Information
dominate over the mesoscale urban heat island.
More work is needed on these important scale
effects.
dry and dusty urban atmosphere, where there is
an ample debris supply, leads to general urban
airflows of only 5m s
-1
being annoying, and those
of more than 20m s
-1
being dangerous.
3 Modification of surface
characteristics
Moisture
The absence of large bodies of standing water in
urban areas and the rapid removal of surface
runoff through drains reduces local evaporation.
The lack of an 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 percent 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. Ground-based weather radar has been
used in a study of Atlanta, Georgia. It is fairly
certain that urban areas in Europe and North
America are responsible for local conditions that,
in summer especially, can trigger excesses of
precipitation under marginal conditions. Such
triggering involves both thermal effects and the
increased frictional convergence of built-up areas.
European and North American cities tend to
record 6-7 percent more days with rain per year
than their surrounding regions, giving a 5-10
percent increase in urban precipitation. Over
southeast England during 1951-1960, summer
thunderstorm rain (which comprised 5-15
percent of the total precipitation) was especially
concentrated in west, central and south London,
and contrasted strikingly with the distribution of
Airflow
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-center wind speeds are usually at least 5
percent 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 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 1961-1962 at Heathrow Airport (open
country within the suburbs) was 2.9m s
-1
,
compared with 2.1m s
-1
in central London. The
comparable figures at night (01:00 hours) were
2.2m s
-1
and 2.5m 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 roughness and by the channeling 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
