In this study, the temperature dependence of atmospheric moisture lifetime is investigated using the European Centre for Medium-Range Weather Forecasts 40-year Reanalysis (ERA40) dataset. ERA40 shows that atmospheric moisture lifetimes can vary by greater than a factor of 4 within the column. Globally, the shortest lifetimes occur at temperatures of about -40¿C, with mean lifetimes of just over a day at this level. Lifetimes derived independently from aircraft microphysical measurements in cirrus clouds confirm the rapid loss of moisture at about -40¿C. This can be understood to result from changes in the rate of nucleation and size distribution of ice particles with decreasing temperature. A similar temperature dependence is observed in water vapor scale height, with relative humidity increasing with decreasing temperature below about -40¿C. The variability of atmospheric moisture lifetime with temperature is found to be a fundamental microphysical control on the distribution of water vapor.