Geoscience Reference
In-Depth Information
climate change. Many weather observation sites were
originally located on the edge of cities. As the cities have
grown, so the urban fabric has spread around the weather
station, causing an increase in temperature, not through
any change in climate but because the local environment
has changed. This feature has to be allowed for in any
study of long-term climate change.
Cloud and precipitation in cities
Most of the climatic changes brought about by urbaniza-
tion have been well documented. They are summarized
though the decrease in the use of coal has led to smaller
modifications in insolation, pollutants and fogs. The
increase in cloud and precipitation over cities was one
aspect which took some time to prove. It was American
work, especially on St Louis, which confirmed the urban
effect conclusively. There appear to be multiple causes of
the increases in cloud cover and precipitation. Added
heating of the air crossing the city, increases in pollutants,
the frictional and turbulent effects on air flow and altered
moisture all appear to play a role. The confluence zones
induced by these urban effects may lead to the preferential
development of clouds and rain. Which factors become
dominant in a particular storm varies according to the
nature of the air circulation over the city on that day. As
the effects are less noticeable in winter than in summer,
it follows that it is the natural, not the artificial, heating
effects which are most important, though the way in
which the summer atmosphere responds to the urban
surface is also significant.
As the degree of urbanization has increased so an ever
greater number of people are affected by an urban climate.
Apart from the more obvious effects of pollution, wind
and warmth, few people may realize that their urban area
has changed other aspects of the climate. The nature of
the urban area represents an extreme example of the way
in which human modification can change the climate
near the ground. It is also causing problems in studies of
So far, all examples quoted have assumed that the ground
surface is almost flat. In reality few areas of the world are
so level that the effect of topography can be ignored. The
reason we need to know more about the topography is that
slopes modify how much short-wave radiation reaches the
surface. We saw earlier that the maximum intensity of
radiation is received when the angle between the surface
and the sun's rays is 90
. If a horizontal surface is tilted so
that it becomes at right-angles to the sun's rays the amount
of radiation received increases. This factor is exploited by
sunbathers, who can tilt the angle of their reclining seats
to achieve maximum heat input. If it were the only factor,
calculating the new input for a slope would be easy.
However, while the slope remains constant, the sun is
continuously changing its position in the sky throughout
the day and throughout the year. Slopes, unlike sun-
bathers, cannot adjust their position. Consequently a slope
that receives maximum intensity at one time on a certain
day of the year may be in shadow at other times.
Effects on the radiation balance
As the movement of the sun across the sky is known, it is
possible to calculate the intensity of short-wave radiation
falling on a slope of any combination of gradient and
orientation (azimuth) for clear skies. More frequently we
are interested in the total radiation rather than the
intensity but even this problem has been overcome using
computers. A computer program can be devised to
calculate the intensity of radiation on the surface for any
particular time and slope. So, for the start of the program,
radiation intensity is determined for sunrise, depending
upon such factors as latitude, time of year, altitude and
atmospheric transmission. Then the computer calculates
the sun's position in the sky, say one minute later, works
out the new radiation intensity and adds its value to the
previous total. This is continued until sunset or until the
sun drops below the horizon (
Figure 8.11
).
We then have
the daily total of short-wave radiation based on intensity
values every minute. The contribution from diffuse
Table 8.4
Effects of urbanization on climate: average
rural conditions
Measure
Annual
Cold
Warm
season
season
Pollution
+500
+1000
+250
Solar radiation
-10
-15
-5
Temperature
+2
+3
+1
Humidity
-5
-2
-10
Visibility
-15
-20
-10
Fog
+10
+15
+5
Wind speed
-25
-20
-30
Cloudiness
+8
+5
+10
Rainfall
+5
0
+10
Thunderstorms
+15
+5
+30
Note: Temperature is expressed as a difference only, not as a
percentage.
