Geoscience Reference
In-Depth Information
The complex nature of the urban modification of
the heat budget is demonstrated by observations made
in and around the city of Vancouver, Canada. Figure
12.24 compares the summer diurnal energy balances
for rural and suburban locations. Rural areas show
considerable consumption of net radiation (
R
n
) by
evapotranspiration (
LE
) during the day, giving lower
temperatures than in the suburbs. While the suburban
gain of net radiation is greater by day, the loss is greater
during the evening and night due to release of turbulent
sensible heat from the suburban fabric (i.e.
wall receives the first radiation in the early morning,
reaching a maximum at 10:00 hours, but being totally
in shadow after 12:00 hours. Total
R
n
is low because
the east-facing wall is often in shadow. The street level
(i.e. canyon floor) is sunlit only in the middle of the day
and
R
n
and
H
dispositions are symmetrical. The third
component of the urban canyon total energy balance is
the west-facing wall, which is a mirror image (centred
on noon) of that of the east-facing wall. Consequently,
the symmetry of the street-level energy balance and
the mirror images of the east- and west-facing walls
produce the symmetrical diurnal energy balance of
R
n
,
H
and
S
negative).
The diurnal energy balance for the dry top of an urban
canyon is symmetrical about midday (Figure 12.25C),
and two-thirds of the net radiation is transferred into
atmospheric sensible heat and one-third into heat
storage in the building material (
∆
S
observed at the canyon top.
The thermal characteristics of urban areas contrast
strongly with those of the surrounding countryside; the
generally higher urban temperatures are the result of
the interaction of the following factors:
∆
S
). Figure 12.25A-B
explains this energy balance symmetry in terms of the
behaviour of its components (i.e. canyon floor and east-
facing wall); these make up a white, windowless urban
canyon in early September aligned north-south and
with a canyon height equal to its width. The east-facing
∆
1
Changes in the radiation balance due to atmospheric
composition.
2
Changes in the radiation balance due to the albedo
and thermal capacity of urban surface materials, and
to canyon geometry.
3
The production of heat by buildings, traffic and
industry.
4
The reduction of heat diffusion due to changes in
airflow patterns caused by urban surface roughness.
5
The reduction in thermal energy required for
evaporation and transpiration due to the surface
character, rapid drainage and generally lower wind
speeds of urban areas.
Consideration of factors 4 and 5 will be left to D.3 (this
chapter).
a Atmospheric composition
Air pollution makes the
transmissivity
of urban
atmospheres significantly lower than that of nearby rural
areas. During the period 1960 to 1969, the atmospheric
transmissivity over Detroit averaged 9 per cent less than
that for nearby areas, and reached 25 per cent less under
calm conditions. The increased absorption of solar
radiation by aerosols plays a role in daytime heating of
the boundary layer pollution dome (see Figure 12.23A)
but is less important within the urban canopy layer,
which extends to mean rooftop height (see Figure
12.23B). Table 12.2 compares urban and rural energy
budgets for the Cincinnati region during summer 1968
Figure 12.24
Average diurnal energy balances for (A) rural and
(B) suburban locations in Greater Vancouver for thirty summer
days.
Source
: After Clough and Oke, from Oke (1988).
