As sound waves traverse in clouds or foggy air the temperature change, which accompanies the pressure variation, would make the water molecules at the droplet's surface to evaporate or those in its close surroundings to condense according as there is condensation or rarefaction respectively. In general, the pressure waves always pro-pagate ahead of the density waives since a finite time is required for this process to take place. However at very high frequencies, the periodic pressure varies too fast for this process to follow, the situation is essentially "dry adiabatic". Only at very low frequencies is the term "relaxation time" of any significance, so the "dispersion" and "absorption".This problem has been handled, in the present paper, in some respect in an analogous way to that in the collision theory of molecular absorption of sound. The previous calculations due to Oswatitsch have been found to be erroneus, and special attention has been given to each of the following:(1) Relation between air density and particle concentration;(2) Relation between the change of the amount of liquid water and the amount of vapor;(3) Derivation of the equation of growth of droplet; and(4) Diffusion and heat conduction coefficients.