Geoscience Reference
In-Depth Information
degree days. This quantitative index is defined
in the United States with respect to daily mean
temperatures above or below 18°C (65°F).
Heating (cooling) degree days are summations of
the negative (positive) differences between the
mean daily temperature and the 18
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 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, conse-
quently, 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 shortwave radiation, as
well as a reduced 'sky view' (proportional to the
areas of the hemisphere open to the sky), which
decreases the loss of infrared 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 (0-1.0) and the
maximum urban-rural temperature difference.
The difference is 10-12
C base.
Heating degree days are accumulated from 1 July
to 30 June and cooling degree days from 1 January
to 31 December.
°
Atmospheric composition
Air pollution makes the
transmissivity
of urban
atmospheres significantly lower than that of
nearby rural areas. During the period 1960-1969,
the atmospheric transmissivity over Detroit
averaged 9 percent less than that for nearby areas,
and reached 25 percent less under calm condi-
tions. 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 under anticyclonic
conditions with <3/10 cloud and a wind speed of
<2m s
-1
. The data show that pollution reduces
the incoming shortwave radiation, but a lower
albedo and the greater surface area within urban
canyons counterbalance this. The increased urban
Ln
at 12:00 and 20:00 LST is largely offset by
anthropogenic heating (see below).
°
C for a sky view factor of
0.3, but only 3
°
C for a sky view factor of 0.8-0.9.
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
25W m
-2
in Europe, compared with similar
heat production from large cities.
Figure 12.26
Table 12.2
Energy budget figures (W m
-2
) for the Cincinnati region during the summer of 1968
Area
Central business district
Surrounding country
Time
08:00
13:00
20:00
08:00
13:00
20:00
Shortwave, incoming (Q+ q)
288*
763
-
306
813
-
Shortwave, reflected [(Q+ q)a]
42†
120†
-
80
159
-
Net longwave radiation (L
n
)
-61
-100
-98
-61
-67
-67
Net radiation (Rn)
184
543
-98
165
587
-67
Heat produced by human activity
36
29
26‡
0
0
0
Source: From Bach and Patterson (1966).
Notes: *Pollution peak. †An urban surface reflects less than agricultural land, and a rough skyscraper complex can absorb
up to six times more incoming radiation. ‡Replaces more than 25 percent of the longwave radiation loss in the evenings.
