The video shows the updraft behind high-rise building. The wind coming from left causes flow separation behind the buildings. In the separation, tracer gas (or surface emitted pollutants) are transported upwards by the flow and thus decreasing surface concentrations a bit. in meteorological terms, air exchange between the obstacle layer and the above-rooftop flow is increased.

The experiment was performed in the large Goettingen-type wind tunnel of the Theodore von Kármán Wind Tunnel Lab (www.karman-wtl.com) in summer 2007.