The extensive air-exchange system of Berlin consists primarily of three components:
Air conducting and ventilation pathways,
orographically-thermally induced, extensive cold air outflows,
thermally induced, line-like cold air pathways.
The air conducting and ventilation pathways, important for Berlin, follow the valleys of Havel, Dahme and Spree. They are significant mainly in case of allochthonous weather conditions, in which regional wind systems are formed because of more or less extensive air pressure differences. In Berlin, these weather conditions occur in the long-term mean (2001-2010), depending upon the station observed, between 61.9 % (Berlin-Grunewald) and 91.5 % (Berlin-Dahlem) (SenStadtUm 2015a). In doing so, western wind directions are prevalent. In the valleys, the introduced cold air is channelised, accelerated and transported in this way to the inner city having comparatively weaker winds (“jet effect”). To be able to make an optimum use of this phenomenon, the shore areas should be kept free and the development should be kept open in the transition areas to the water bodies.
Autochthonous weather conditions without (or with only weakly pronounced) higher-level wind systems occur rather rarely in Berlin (8.5 % – 38.1 % of the yearly hours). However, they are normally connected with stronger stresses for the health of the urban population, because the transportation of air pollutants is inhibited because of inversions and there is a characterisation of the urban heat island. For these weather situations, local, thermal and/or orographically induced cold air flows and floor wind systems take up the supply of the city with cold/fresh air.
The thermally-orographically induced cold air outflow is to be traced back to relief differences, which in the morning hours leads to an outflow of the cooling air parallel to the slope. The prerequisite for cold air outflow volume relevant for planning is an extensive slope inclination of > 1 %, which should also be assigned in the direction of a (thermally stressed) settlement area. Grunewald has the biggest, area-wise cold air outflow potential. The cold air can outflow here to almost 3,500 ha. The adjoining residential development benefits directly from this, in particular, in the northern and eastern part.
Purely thermally induced cold air pathways are, on the other hand, more frequent and also distributed more homogeneously over the city area. The reason for these is the small-scale succession of local high and low pressure area within Berlin during the night hours of autochthonous weather conditions and they see to it that air rising over the warm, densely built settlement areas is replaced at ground level by comparatively cooler air masses from their surroundings, especially larger green/open spaces. In particular for the inner city area, they represent the most important relief effects.
Demarcating their effective areas against one another and against those of the other components of the air-exchange system based on clearly defined areas is not possible without further model and technical measurement analyses owing to spatial overlaps. However, the core areas of the individual pathways can be demarcated spatially on the basis of modelling and approximately balanced and compared. The greenways are suitable to a large extent as core area of the thermally induced pathway type. They not only transport further the cold air generated in the outer area, but also enrich the air stream with additional volume of cold air. Relevant quantities of cold air can also be transported to the city over broad streets. The pathways with air-hygiene stress must be differentiated here from the unstressed pathways (VDI 2015).
The identification of the pathways and their corridors was done manually as an advisory estimate and is based on the characterisation of the autochthonous flow field of the FITNAH simulation carried out. In doing so, the demarcation of the pathway corridors is not for clearly defined areas and in the actual planning case (e.g. a construction project) requires at least one additional expert assessment.
For the city area of Berlin, a total of 21 pathways were identified (see Figure 7). Their core regions include a total area of around 1,250 ha, which is the same as 1.4% of the complete city area. Each pathway represents a central component of the air-exchange system of Berlin. For this reason, all structural obstacles are to be avoided, which could cause a cold air jam. The basic objective should be to maintain the portion of green and open areas. In case of a construction, the construction height should be kept as low as possible and the new structure should be aligned longitudinally to the pathway. Perimeter developments are to be avoided completely.