In this work, the liquid flowrate in an overflowing pneumatic foam is studied as a function of environmental humidity. It is found that in general, liquid flowrate is greater when the relative humidity of the air above the foam is high. This observation is attributed to bubble coalescence on the surface of the foam. Because the evaporation rate is diminished when the relative humidity is high, less bubbles collapse on the surface of the foam. Because no experiments have been conducted on mineralised flotation froths, a dependency on the operation of froth flotation devices on environmental humidity cannot be directly asserted; although the effect is so important in demineralised foams that it should not be overlooked in the context of flotation. The authors know of no laboratory or plant studies of froth flotation that have controlled or measured the environmental humidity. (C) 2010 Elsevier B.V. All rights reserved.