CHARACTERISTICS AND FORMS OF THE URBAN CLIMATE
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 2/9 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 (ROBEL et al., 1978).

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
2/10) (GANDEMER, 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.

Erstere, zu denen Hangwinde sowie Berg- und Talwinde gehören, sind stark durch das vorhandene Relief geprägt. Figure 2/11 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.
Figure 2/12 and 2/13 show an increasing of the cool air flow in Freiburg i. Breisgau. (RICHTER u. RÖCKLE, 2003)

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))

For the wind speed, the following calculations be valid:

Meters per second	1 m/s	= 1,943 Knots = 3,600 km/h
Kilometers per hour	1 km/h	= 0,540 Knots = 0,278 m/s
Knots (nautical miles per hour)	1 kn	= 1,852 km/h = 0,515 m/s

Table 2/1: Conversion table for the wind speed