The negative impacts of urban traffic growth are well known: congestion, increased air pollution, more traffic noise, etc. Humans most easily appreciate what their senses can directly sense; they can readily see, and even hear, traffic congestion, and can hear traffic noise, but inevitably they are much less aware of odourless, invisible, silent air pollution. Therefore, quite naturally, people - whether the general public or transport professionals - struggle to visualise air pollution from traffic, and will find it particularly difficult to visualise changes in air pollution levels resulting from changes in traffic conditions due to the implementation of urban transport schemes. Hence there is always a risk of the air quality impacts of potential urban transport schemes being under-appreciated. Using a specific area of the UK city of Nottingham as a case study, research has been undertaken to integrate an air pollution dispersion model for the pollutant NO2 and a 3D digital city model of the case study area, and then to represent the modelled NO2 concentrations at various heights above the ground as, firstly, a 3D point array, and then, secondly, as 3D volumetric clouds, using the analogy of people's perceptions of grey clouds in the sky as representing undesirable weather.