RECOMMENDATIONS FOR PLANNING
6.2.1 Cold Air Production

Proportional to their size, green open spaces – that is, meadows, fields, fallow land, and cultivated areas with low vegetation cover – produce 10 to 12 m3 of cold air per square meter per hour as a result of their nightly cooling. When the cold air does not flow elsewhere, this raises the upper boundary of the cold air layer by 0.2 m per minute. Thus a cold air layer 12 meters thick can arise in the span of an hour. The high cold-air productivity of green open space is connected with the characteristic that cold air flowing away from these areas is only slightly constrained by flow hindrances.

Forest areas likewise function as nightly cold air producers. A forest will cool a larger volume of air than an equivalent area of open space; however, the air does not reach the low temperatures of the open space. The upper surface of the canopy in a deciduous or evergreen forest shields the forest floor from the atmosphere, so that the central layer of the forest is neither as strongly heated during the day nor cools as much during the night as the layer of air above an open area. This balancing effect on the daily variation of air temperature allows forests near urban areas to produce cold air during the day as well for the benefit of the developed area. Forest areas on northern and eastern slopes, which are subject to lesser sun exposure, are especially favorable for daily cooling.

Legal Bases

The graphical and regulatory options discussed in Chapter 6.1.1 come into consideration here in principle. The establishment of areas for agriculture or forests must make sure that land- and forest-related economic interests are dealt with. Areas in construction sites to be kept free of development, as a rule, can only be established for reasons of urban planning. These include the layout of the local and landscape images, the securing of views on slopes, and the requirements of sufficient ventilation. The concretely-established land use on these areas is then directed towards the respective urban development purpose.