Reply to A. G. C. A. Meesters et al.'s comment on "Biotic pump of atmospheric moisture as driver of the hydrological cycle on land"

Abstract. Condensation removes water vapor molecules from the gas phase and reduces the weight of the air column. This disturbs hydrostatic equilibrium and makes air circulate under the action of the recently described evaporative force. Meesters, Dolman and Bruijnzeel (2009) criticized the physical bases of the new circulation driver with a major claim that the ascending air motions induced by the evaporative force should rapidly restore the hydrostatic equilibrium and become extinguished. Here we respond that in fact these air motions sustain the disequilibrium of air pressure through the reduction of the weight of the air column via condensation that continuously occurs as the ascending moist air cools. In the traditional meteorological paradigm condensation is primarily considered in terms of the effect it has, via latent heat release, on air density, while its immediate effect on the weight of air column is not accounted for. The critique of Meesters et al. is therefore informative in highlighting the traditional lines of thought that should be re-visited to incorporate the new physical knowledge. Such an effort is arguably worthy of undertaking as the evaporative force concept bears tangible potential for solving some of the key problems that are challenging modern atmospheric science.