A thin water film was allowed to run down the outside of a vertical tube. Air was blown upwards through a concentric orifice surrounding the tube, thereby causing the air to accelerate over the surface of the water film. The resulting pressure gradients in the airstream caused local thickening of the film, and at a sufficient airflow these pressure gradients balanced the gravity forces acting on the film, and the large wave shown in figure 1a was formed. In the theoretical calculations, irrotational motion in the airstream was assumed, and mass accelerations in the water film were neglected. The simplest theory, neglecting the effect of the water film on the airstream, gave an estimate of the critical airflow which was too high. More complex calculations, allowing for the effect of the water wave on the airflow and vice versa, gave good agreement with experiment. Preliminary experiments, using a simple model of a packed tower, suggest strongly that the wave formation described above is responsible for the initial breakdown of film flow in a packed tower, known to chemical engineers as the 'loading point'.