It is experimentally shown that the stability of non-overflowing pneumatic foams, such as those used in the Bikerman foam stability test, is strongly dependent upon the gradient of humidity within the freeboard of the column. Growing foams can be prevented from exhibiting collapse, even at surfactant concentrations significantly lower than the critical micelle concentration, by saturating the air above the foam surface thereby preventing liquid evaporation. These observations have important implications for the performance of flotation devices, as well as instruments designed to measure the stability of aqueous foams. Meteorological data for two mining sites in Australia throughout a single year shows the variability of humidity, and therefore stability, of flotation froths. (C) 2009 Elsevier B.V. All rights reserved.