The McCook Field high-speed wind tunnel is an apparatus for causing a current of air to flow past any stationary object, such as a model aerofoil. The aerodynamic forces produced on the model are measured and can be interpreted for full-scale use. A means of actually visualizing the air during these experiments is therefore of the greatest interest. The method for producing visualization involves condensation of the moisture contained in the air. This moisture, when turned into visible vapor by suitable means, takes up shapes dependent upon the motion and pressure gradient of the air. Flight vortices become readily visible by the McCook method and can be photographed with the aid of searchlights.