Thin-walled, storage tanks are prone to vacuum collapse because of the development of a net external pressure. Condensation of steam inside the vessel can cause the rapid development of this mode of failure. External atmospheric conditions also affect the process. An expression to predict the time taken for collapse to occur is developed. The formula is validated by a series of experimental tests on a variety of laboratory scale drums and at a number of different air flow conditions. A parameter study indicates the relative importance of the input variables used.