Windward ventilation increases ventilation rates in greenhouses compared to leeward ventilation. However, in areas such as the Canary Islands tomato greenhouses use to be large and, furthermore, are located in areas with hardblowing winds which involves wind damage to crops (through side vents) and to windward roof vents. For this reason leeward ventilation is preferred to windward ventilation. Nevertheless some improvements are needed for large span greenhouses with leeward ventilation in order to eliminate hot areas and thus increase temperature uniformity. For this purpose CFD simulations were run on a experimental greenhouse with lee ventilation. CFD simulations showed that there was an area of negative air pressure near the gutter of the first span. Therefore a ventilator was built on this location in order to act as an air outlet. In this report we present the results of the study of this additional vent. Three sonic anemometers were placed outside and inside the greenhouse in different positions. The experimental data agreed with the general flow pattern shown by CFD simulations: the additional ventilator acted as an efficient air outlet and the internal air movement was as predicted by simulations. Preliminary analysis has shown a good correlation between outside wind velocity and the velocity outlet through the additional vent. The correlation between outside wind speed and air velocity in the centre of the greenhouse was lower than that for the additional ventilator.