Building ventilation potential and pollutant dispersion are greatly affected by the airflow pattern in street canyons. Two reasons for airflow in a street canyon exist: wind washing and buoyancy due solar radiation on one of the facades of the street canyon. The importance of the buoyancy effect inside street canyons can be characterised by the Froude (Fr) number which is directly related to the air speed (Uref) on top of the boundary layer and inversely related to the temperature difference between the heated wall and the ambient air temperature and to the height of the street canyon. The wind effect on the airflow pattern inside courtyards and on thermal comfort in the surrounding rooms has been studied extensively by Tablada (2006) (1). To study both wind and buoyancy effects, experiments were performed by Kovar-Panskus et al. (2002) (2) in a wind tunnel on a square street canyon. One street façade was heated to obtain a uniform temperature over its whole surface. The airflow pattern inside the street canyon was measured for four different Froude numbers: 2.03, 1.17, 0.73 and 0.27. They found a marked transition from a single dominant vortex, existing at higher Fr values (dominant wind effects), to a combination of vortices due to buoyancy and wind effects. Below Fr ≈ 1, the main vortex shifts to the top of the canyon and a lower counter-rotating vortex develops against the heated wall. The objective of this study is to analyse in more detail the relative importance of wind versus thermal stack on the type of airflow pattern. This is done by further analysing the airflow pattern in a square canyon by numerical calculations using Computational Fluids Dynamics (CFD) for the Kovar-Panskus experiment configuration. Using CFD for an array of wind velocities (ranging from 0.1 to 10 m/s) and surface temperature of one street façade (temperature differential with ambient air varying from 5 to 93K), 42 simulations were performed (Froude numbers ranging from Fr = 0.0115 to Fr = 2146). The results show that for high wind velocities (Fr>30), at any temperature differential, one single main vortex is formed, highlighting the predominant role of wind in the airflow pattern in the street canyon. At lower velocities, the airflow patterns depend on temperature. At high temperature differential, a vortex is formed on the full height of the street façade, compressing the wind induced vortex to one side of the canyon. At low temperature differential, the two vortex touch the heated façade, in the upper part is the wind-induced vortex, in the lower part is the thermal stack-induced vortex. Results are presented in function of the Froude number. (1) Tablada A. Shape of new residential buildings in the Historical Centre of Old Havana to favour natural ventilation and thermal comfort. PhD Thesis, Department of Architecture, Urbanism and Planning, Katholieke Universiteit Leuven. 2006. (2) Kovar-Panskus A, Moulinneuf L, Savory E, Abdelqari A, Sini JF, Rosant, JM, Robins A, Toy N. A wind tunnel investigation of the influence of solar-induced wall-heating on the flow regime within a simulated urban street canyon. Water, Air, & Soil Pollution: Focus 2, 2002, pp 555-571. doi: 10.1023/A:1021345131117