1.4. Wind

The city induces a change in the wind fields both in direction and intensity. Irregularities due to buildings of different heights lead to an increase in friction which is due to an increase in the roughness of the surface. As a result, the wind slows near the surface and the speed decreases (Figure 12). In addition, the frequency of windless situations (calm) is also increased. For example in Strasbourg, the average annual speed measured above the average level of the roofs (city center) is 1.5 m/s while outside the city it is 2.9 m/s (Airport), this represents a reduction of almost 50% of the mean wind speed.

Turbulence in town increases (more swirling wind). Rather than going straight ahead, the air describes vortices (circles). This turbulence is mainly of mechanical origin, that is to say that it comes from the fact that the air rubs on the rough ground (it slows down) and it collides with obstacles which forces it to suddenly change its trajectory (Figure 13).

Urban morphology and therefore the way in which the obstacles are arranged, plays a preponderant role for the wind in urban canyons (Figure 14).