Geoscience Reference
In-Depth Information
building geometry. Table 12.2 shows the relatively
high heat absorption of the city surface. A problem of
measurement is that the stronger the urban thermal
influence, the weaker the heat absorption
at street level
,
and, consequently, observations made only in streets
may lead to erroneous results. The geometry of urban
canyons is particularly important. It involves an increase
in effective surface area and the trapping by multiple
reflection of short-wave radiation, as well as a reduced
'sky view' (proportional to the areas of the hemisphere
open to the sky), which decreases the loss of infra-red
radiation. From analyses by T. R. Oke, there appears to
be an inverse linear relationship on calm, clear summer
nights between the sky view factor (0 to 1.0) and the
maximum urban-rural temperature difference. The
difference is 10 to 12°C for a sky view factor of 0.3, but
only 3°C for a sky view factor of 0.8 to 0.9.
c Human heat production
Numerous studies show that urban conurbations now
produce energy through combustion at rates comparable
with incoming solar radiation in winter. Solar radiation
in winter averages around 25 W m
-2
in Europe, com-
pared with similar heat production from large cities.
Figure 12.26 illustrates the magnitude and spatial scale
of artificial and natural energy fluxes and projected
increases. In Cincinnati, a significant proportion of the
energy budget is generated by human activity, even in
summer (see Table 12.2). This averages 26 W m
-2
or
more, two-thirds of which was produced by industrial,
commercial and domestic sources and one-third by cars.
In the extreme situation of Arctic settlements during
polar darkness, the energy balance during calm condi-
tions depends only on net long-wave radiation and heat
production by anthropogenic activities. In Reykjavik,
Iceland (population 100,000) the anthropogenic heat
release is 35 W m-
2
mainly as a result of geothermal
pavement heating and hot water pipelines.
Figure 12.25
Diurnal variation of energy balance components
for a N-S-oriented urban canyon in Vancouver, British Columbia,
having white concrete walls, no windows, and a width/height ratio
of 1:1, during the period 9 to 11 September 1973. (A) The
average for the E-facing wall. (B) The average for the floor. (C)
Averages of fluxes through the canyon top.
Source
: After Nunez and Oke, from Oke (1978).
under anticyclonic conditions with < 3/10 cloud and a
wind speed of <2 m s
-1
. The data show that pollution
reduces the incoming short-wave radiation, but a lower
albedo and the greater surface area within urban
canyons counterbalance this. The increased urban
L
n
at
12:00 and 20:00 LST is largely offset by anthropogenic
heating (see below).
d Heat islands
The net effect of urban thermal processes is to make city
temperatures in mid-latitudes generally higher than in
the surrounding rural areas. This is most pronounced
after sunset during calm, clear weather, when cooling
rates in the rural areas greatly exceed those in the urban
areas. The energy balance differences that cause this
effect depend on the radiation geometry and thermal
b Urban surfaces
Primary controls over a city's thermal climate are the
character and density of urban surfaces; that is, the total
surface
area of buildings and roads, as well as the
