Environmental Engineering Reference
In-Depth Information
urban area can produce major changes. The parkland, especially if irrigated, has different
heat capacities, albedos, moisture levels and emission temperatures from the surrounding
buildings, giving slightly lower day and nighttime temperatures. The advantages of these
'urban lungs' extend well beyond their aesthetic appeal, especially during hot summer
weather.
Even when winds are not light, the presence of the urban structure tends to slow air
movement down. Wind records from city-centre sites show lower average speeds than
suburban or rural locations near by, although the degree of gustiness may be higher,
especially in summer. As the air flows over the very irregular suface of a city, friction
with the buildings retards the wind in the lowest layers (Figure 8.11). The presence of
skyscrapers, however, produces eddies which can cause strong local winds. At street
level these can become quite unpleasant, raising dust, perhaps even rubbish, and making
walking difficult. Quite a few shopping precincts were unpopular with shoppers until the
architects realized that such winds could be a problem and took measures to minimize
their effects.
CLOUD AND PRECIPITATION IN CITIES
Most of the climatic changes brought about by urbanization have been well documented.
They are summarized in Table 8.4. Some of the changes are appreciable, 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.
Figure 8.10
Mean hourly temperature differences between
rural and urban Chicago in relation to cloud cover and wind
