  The physical equations determining the motion of moist atmospheric air in the
presence of condensation remain controversial. Two distinct formulations have
been proposed, published and cited. The equation of Bannon [2002, J. Atmos.
Sci. 59: 1967--1982] includes a term for a "reactive motion" that arises when
water vapor condenses and droplets begin to fall; according to this term the
remaining gas moves upwards so as to conserve momentum. In the equation of
Ooyama[2001, J. Atmos. Sci. 58: 2073--2102] the reactive motion term is absent.
Both equations contain a term for condensate loading, but in the formulation of
Ooyama [2001] there are two additional terms. In some modern nonhydrostatic
models of moist atmospheric circulation, however, formulations have been mixed.
Here we examine the contrasting equations for the motion of moist air. We
discuss inconsistencies in the application of Newton's second and third laws to
an air and condensate mixture. We show that the concept of reactive motion in
this context is based on a misunderstanding of the conservation of momentum in
the presence of a gravitational field: such a reactive motion does not exist.
We show that the "mixed" equation used in some models is not physically
consistent either. We examine why consideration of total momentum, that is air
and condensate combined, has been misleading in the search for valid equations
of motion in the presence of phase transitions.
