The performance of natural ventilators subjected to the action of a free stream are experimentally investigated by using the method of Baturin. To analyse experimental data a mathematical model of the extraction characteristics is introduced. The model has been obtained from considerations of natural ventilator behaviour under extreme conditions within the normal operating range (zero external free stream and zero mass flow through the ventilator) and includes a correction term which accounts for the interference between the external wind and the flow through the ventilator. In this model ventilator performances are explained in terms of only four parameters which in turn are easily calculated from experimental results. Finally, the steps followed to enhance the performance of an existing ventilator are shown as an illustration of the use of the model here presented.