RECOMMENDATIONS FOR PLANNING
6.2.2 Fresh Air Supply

When cold-air production areas lie in or near the catchment areas of valleys and vales leading to developed areas, natural paths for the supply of fresh cold air are thereby provided, since cooler air continually flows towards lower topographical areas. The intensity of flow depends upon the size of the catchment area, the angle of slope inclination, the width of the valley, and the degree of freedom from obstruction. Cleaner fresh air can only be supplied by local air flows when the environment (in the form of the surrounding area and larger parks in cities) is intact and not excessively burdened by pollution.

Potential hindrances for cold air flow include: Narrowed sections of valleys, dams, noise protection barriers or walls, rows of trees perpendicular to valley alignment, and blockages from larger buildings or entirely enclosed urban structures. Cold air stalls when it encounters hindrances; its temperature drops strongly and air exchange is reduced in the area in front of and behind the hindrance (leading to danger of early or late frost or strengthened fog production). It is mostly the emanation of heat above built-up areas that leads to a reduction in the range and effect of cold air flow.

Also to be considered under this aspect are sections of road that run in the direction of cold air flow, even when they can be favored as hindrance-free corridors for air exchange near the ground. In the case of roads perpendicular to valley alignment, the thermal behavior of the narrow strip of road plays a subordinated role, whereby the hindrance effect of relief-altering measures (e.g. road dams and noise protection edifices) dominates.

Cross-municipal planning must take care that regional cold-air production areas can fulfill their special function. This includes the restriction of development on open areas and, where necessary, artificial foresting.

As long as slopes angled towards a city are already forested or foreseen for forestation, a sufficiently large surface in each case is to be kept open between the edge of the forest and the built-up area in order to ensure the daytime flow of especially valuable cold air out from the forest.

Valleys and other gorge-like features important for the supply of fresh air are to be maintained as fresh-air corridors and should be kept free of development to a particularly large extent. Perpendicular-standing buildings in lower valley areas represent a substantial hindrance. Large changes of elevation in the lengthwise profile of a valley are also to be avoided. Groups and rows of trees perpendicular to valley alignment are likewise obstructive for air exchange near the ground. As far as necessary in individual cases, a hindrance of the cold air flow can be overcome by thinning or clearing these trees. If necessary, a desired change in air flow direction can be accomplish by foresting in the shape of a cork at a different location. Urban structures should not intrude into valleys.

Legal Bases

The reservation of fresh air corridors can be secured through corresponding land use depictions and regulations in zoning codes and plans (e.g. as green spaces, sport and recreation facilities, or areas for agriculture). The establishment of minimum sizes for building lots (§ 9 (1) 3. BauGB) along the lines of Chapter 6.1.3 can also support the ventilation of an area.

In the explanatory report accompanying a land use plan (§ 5 (5) BauGB) or in the basis for a site plan (§ 9 (8) BauGB), the local climatic importance of the affected areas to provision of fresh air for developed locales is particularly to be addressed.