Heat island effect and air quality are major issues in big cities and the shape ofthe street canyons is directly related with these problems. The only presence the buildings modifies the temperature in their surroundings because of the direct interference with radiation, moisture and wind. However, the main reason why buildings affect temperature is because of the interference withthe airflow. Therefore, a study of the air through the street canyons must be done. This computational fluid dynamics (CFD) study will be made with open source programs. To validate our model a 2D case will be solved and the results will be compared with the experimental data from the Bibliography.Once the model is validated, a 3D geometry will be solved in order to study the case proposed. In this late geometry the fact that the airflow passes through the buildings sideways will be considered. Then, the effect of the presence of buildings and the geometry of street canyons on the temperatures in cities will be discussed.