This work presents recent flow measurements in a homogeneous urban geometry made with the use of Particle Image Velocimetry. The generated data is used to examine the flow and exchange processes that are established, and thereby to deduce the ventilation capacity of the urban area. The urban geometries addressed have the form of regularly spaced street-canyon arrays of different packing densities, ranging from ëp = 0.30 to 0.67, thus reflecting typical European and American cities. The flow measurements reveal that the dominant flow feature within the street canyon cavity is a recirculation cell, while vertical flow and momentum exchange processes occur at the building roof top level. The mass exchange process at the building roof top level was characterized and quantified by a mean exchange velocity, which was deduced from the measured mean exchanged mass flux. The exchange velocity values normalized over the mean bulk velocity were found to range from 0.016 for the sparser geometries (ëp = 0.33) to 0.007 for the denser geometries (ëp = 0.55). This result corresponds to a reduction in the “ventilation” capacity (or breathability) of the denser city by more than 50%. Finally, by fitting appropriate logarithmic velocity profiles in the flow, appropriate surface roughness length and dispalcement lengths are deduced for the corresponding packing densities.