Studies on air circulation became of great importance in recent years, since are crucial for the energy consumption of buildings, for the pollutant dispersion within cities and for the good comfort conditions for the pedestrians and the habitants. The semi-empirical model developed in this study aims to accurate wind speed computation inside street canyons. In the framework of the Urbvent European Research project, an extended experimental campaign took place in five different urban street canyons in the centre of Athens during the summer of 2001. Wind speed and direction measurements took place, every 30 seconds at several heights within the canyon, as well as and above on the rooftop of the buildings. Wind speed measured near the building facades using the same time interval. The experimental campaign lasted three days per canyon. The experimental data were grouped into different categories based on the measurements of the wind speed above the rooftop of the buildings and the incidence angle on the canyon axis. For all of the derived categories existing algorithms were gathered and grouped into a semi-empirical model. By the use of the above mentioned model wind speed calculated inside the canyons at exactly the same spots where wind speed was measured during the measuring campaign. After comparing between measured and computed wind speed values, derived from the semi-empirical model, we resulted into agreement.