  How phase transitions affect the motion of moist atmospheric air remains
controversial. In the early 2000s two distinct differential equations of motion
were proposed. Besides their contrasting formulations for the acceleration of
condensate, the equations differ concerning the presence/absence of a term
equal to the rate of phase transitions multiplied by the difference in velocity
between condensate and air. This term was interpreted in the literature as the
"reactive motion" associated with condensation. The reasoning behind this
"reactive motion" was that when water vapor condenses and droplets begin to
fall the remaining gas must move upwards to conserve momentum. Here we show
that the two contrasting formulations imply distinct assumptions about how
gaseous air and condensate particles interact. We show that these assumptions
cannot be simultaneously applicable to condensation and evaporation. "Reactive
motion" leading to an upward acceleration of air during condensation does not
exist. The "reactive motion" term can be justified for evaporation only; it
describes the downward acceleration of air. We emphasize the difference between
the equations of motion (i.e., equations constraining velocity) and those
constraining momentum (i.e., equations of motion and continuity combined). We
show that, owing to the imprecise nature of the continuity equations,
consideration of total momentum can be misleading and that this led to the
"reactive motion" controversy. Finally, we provide a revised and generally
applicable equation for the motion of moist air.
