Geoscience Reference
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mean annual total rainfall. During this period,
London's thunderstorm rain was 200-250mm
greater than that in rural southeast England.
Urban areas in the Midwest and the southeastern
USA significantly increase summer convective
activity. Areas in eastern metropolitan Atlanta
received 30 percent more rainfall during days of
mT air in June to August 2002-2006 than areas
west of the city. Both precipitation amount and
frequency were enhanced up to 80km to the east
of the urban core of Atlanta. The enhanced rainfall
was most evident between 19:00 and midnight
LST. More frequent thunderstorms and hail occur
for 30-40km downwind of industrial areas of St
Louis compared with rural areas (
Figure 12.30
).
The anomalies illustrated here are among the best-
documented urban effects. Many of the urban
effects here are based on case studies.
Table 12.3
gives a summary of average climatic differences
between cities and their surroundings.
Thunderstorm
anomaly (> +40%)
Summer total rain
anomaly (> +10%)
H
eavy rain rate
anomaly (> +50%)
Hail anomaly
(> +50%)
Built-up
area
N
0
20
km
Figure 12.30
Anomalies of summer rainfall, rate of heavy
rains, hail frequency and thunderstorm frequency downwind
of the St Louis, MO, metropolitan area. Large arrows indicate
the prevailing direction of motion of summer rain systems.
Source: After Changnon (1979). Courtesy of the American Meteorological
Society.
4 Tropical urban climates
A striking feature of recent and projected world
population growth is the relative increase in the
tropics and subtropics. Today there are 45 world
megacities with more than five million people
inhabitants. By AD 2025 it is predicted that, of
the 13 cities that will have populations in the
20-30 million range, eleven will be in less-
developed countries (Mexico City, São Paulo,
Lagos, Cairo, Karachi, Delhi, Bombay, Calcutta,
Dhaka, Shanghai and Jakarta).
Despite the difficulties in extrapolating
knowledge of urban climates from one region
to another, the ubiquitous high-technology
architecture of most modern city centers and
multi-storey residential areas will tend to impose
similar influences on their differing background
climates. Nevertheless, most tropical urban built
land differs from that in higher latitudes; it is
commonly composed of high-density, single-
storey buildings with few open spaces and poor
drainage. In such a setting, the composition of
roofs is more important than that of walls in terms
Table 12.3
Average mid-latitude urban climatic
conditions compared with those of surrounding rural areas
Atmospheric
carbon dioxide
×
2
composition
sulfur dioxide
×
50-200
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 <5mm
+10%
Source: Partly after the World Meteorological Organization (1970).
