Based on these experiences gained in the New Yorker Subway a comparable measurement program has been developed for the Underground in Dortmund, Germany. The latter clearly differs from the former as follows: 1. Different structure of stations and tunnels, 2. shorter lengths of trains, 3. no air conditioning of the trains, and 4. no overheating effects due to absence of air conditioning in underground areas (rooms for Subway personal) and distant heat transfer systems that heat up tunnel walls (in Dortmund).
Despite these differences, the main common characteristic is that both systems are ventilated pas-sively by entrances/ exits and tunnel openings. Thus, air currents inside the station and tunnel sys-tem of both Underground systems are mainly influenced by effects of regional and local weather conditions (in addition to train movements). The most important are: 1. Temperature/density and humidity gradients between subway and urban atmosphere (upward or downward winds). Here, especially diurnal and seasonal changes are of major importance. 2. Regional wind speed and direction. 3. Microscale/street-level weather conditions, including: wind eddies above the exit/entrance area
As both investigation areas differ greatly due to their climatic background situation (e.g. in their extreme values of climatic variables) distinct differences appear regarding air flow conditions and air exchange with the urban atmosphere, respectively.
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