Geoscience Reference
In-Depth Information
Table 12.3
Average urban climatic conditions compared with those of
surrounding rural areas.
Atmospheric composition
carbon dioxide
2
sulphur dioxide
200
nitrogen oxides
10
carbon monoxide
200(+)
total hydrocarbons
20
particulate matter
3 to 7
Radiation
global solar
-15 to 20%
ultraviolet (winter)
-30%
sunshine duration
-5 to 15%
Temperature winter minimum
(average)
+1 to 2°C
heating degree days
-10%
Wind speed annual mean
-20 to 30%
number of calms
+5 to 20%
Fog
winter
+100%
summer
+30%
Cloud
+5 to 10%
Precipitation
total
+5 to 10%
days with <5 mm
+10%
Source
: Partly after World Meteorological Organization (1970).
than in urban areas due to high rural soil moisture levels
and high urban albedos.
Tropical heat island tendencies are rather similar to
those of temperate cities but are usually weaker, with
different timings for temperature maxima, and with
complications introduced by the effects of afternoon and
evening convective rainstorms and by diurnal breezes.
The thermal characteristics of tropical cities differ from
those in mid-latitudes because of dissimilar urban
morphology (e.g. building density, materials, geometry,
green areas) and because they have fewer sources of
anthropogenic heat. Urban areas in the tropics tend
to have slower rates of cooling and warming than do
the surrounding rural areas, and this causes the major
nocturnal heat island effect to develop later than in mid-
latitudes (i.e. around sunrise (Figure 12.31A)). Urban
climates in the subtropics are well illustrated by four
cities in Mexico (Table 12.4). The heat island effect is,
as expected, greater for larger cities and best exem-
plified at night during the dry season (November to
April), when anticyclonic conditions, clear skies and
inversions are most common (Figure 12.31B). It is of
note that in some tropical coastal cities (e.g. Veracruz;
Figure 12.31A), afternoon urban heating may produce
instability that reinforces the sea breeze effect to
the point where there is a 'cool island' urban effect.
Elevation may play a significant thermal role (Table
12.4), as in Mexico City, where the urban heat island
may be accentuated by rapid nocturnal cooling of
the surrounding countryside. Quito, Ecuador (2851 m)
shows a maximum heat island effect by day (as much as
4°C) and weaker night-time effects, probably due to the
nocturnal drainage of cold air from the nearby volcano
Pichincha.
Ibadan, Nigeria (population over one million;
elevation 210 m), at 7°N, records higher rural than
urban temperatures in the morning and higher urban
temperatures in the afternoon, especially in the dry
season (November to mid-March). In December, the
harmattan dust haze tends to reduce city maximum tem-
peratures. During this season, mean monthly minimum
temperatures are significantly greater in the urban heat
island than in rural areas (March
12°C, but December
only
2°C due to the atmospheric dust effect). In
