| 2. |
Characteristics and Forms of the
Urban Climate . |
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2.5 Wind
A special characteristic of the city is the change in wind patterns with regards to both the direction and the velocity of wind. The larger surface area and irregularity of a built area causes increased friction that reduces wind velocity – up to 30% less on average per year. In particular, the frequency of zero-wind conditions is increased by up to 20%, which leads to a reduction in air circulation and thus hinders the dispersal of pollutants. Figure
2/8 depicts the differences in the wind profiles of
the inner city, the city periphery, and the countryside. In
addition to a reduction in wind velocity as one nears the city
center, one can also notice a greater disturbance of the wind
field in the city center.
At the same time, the
gustiness in cities can also be increased by the formation of
whirlpools next to tall buildings, which leads to the emergence
of drafts near the ground and partially to restrictions on uses
in the areas around the buildings (Figure 2/9, GANDAMER, 1977). The illustration shows
sketches of air flows around buildings that can lead to
whirlpool formation depending on flow conditions. The eddying of
air in the vicinity of buildings exerts particular effects on
the diffusion of particulate matter from fireplaces and other
pollutant sources near the ground. As a rule, the disruption of
the wind field from a building stretches to a distance
corresponding to ten times the building’s height. Local wind systems, which
mainly show up in areas of weak regional winds, can also be of
importance to the movement of air in city areas. These are
distinguished between systems that are dependent on topography
and those that are determined by the built environment. The former of these, which
includes slope winds as well as mountain and valley winds, are
shaped strongly by the topographical relief at hand. Figure
2/10 shows sketches of the circulation pattern of slope,
mountain, and valley winds (LILJEQUIST, 1974). This circulation
is important above all for cities in valleys and
"bowl" or "basin" locations, since it
contributes to the removal of pollutants as well as to the
supply of fresh air. The development of cold air flows, which
typically occur at night close to the ground, depends on the
size of the surfaces that produce cold air and on the incline of
the slope. The latter (corridor
winds) are much more difficult to demonstrate and are much less
pronounced. They are important to cities with a relatively flat
relief and are directed towards the middle of the city (BARLAG, KUTTLER (1991),HUPFER, KUTTLER,
1998, WEBER, KUTTLER (2003)). |
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| Fig. 2/9: Influence of buildings upon air flows, GANDEMER, 1977 | |||
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| Fig. 2/10: Interplay of hillside and valley winds over the course of a day from DEFANT | |||
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| Fig. 2/11a: Flow model for the area of "Stuttgart 21" | |||
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Fig. 2/11b: Cold air flow in
Freiburg 30 minutes after using (RICHTER u. RÖCKLE, 2003) |
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Fig. 2/11c: Cold air flow in
Freiburg 5 hours after using (RICHTER u. RÖCKLE, 2003 |
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