Strong air movements guarantee air exchange in cities, which can lead to the removal of pollutant particles from the air.

In contrast, wind patterns during weather conditions with low winds are more problematic (e.g. during the influence of persistent high pressure). When the vertical air exchange is strongly hindered at the same time, low- or no-exchange weather conditions are present. In these wind patterns, local wind systems supplying fresh air are of exceptional importance.

The lack of air exchange is produced by the presence of a temperature inversion that prevents the vertical removal of pollutants. In such an inversion, the temperature in an air gradient increases with altitude. This means that colder and thus heavier air is concentrated near the ground (Figure 2/12).

Weather conditions with inversions (which can also persist throughout the day) typically appear in winter, as decreased sun elevations and shorter durations of sunshine do not produce a decisive warming of the ground or the layer of air above it.

Basin landscapes and valleys – which also happen to be the preferred locations of urban and industrial development – are especially favorable for the formation of such conditions (Figure 2/20c).

Fig. 2/12: Temperature gradients and pollutant dispersal