The author measured the velocity of the cloud particles falling in free air and obtained the radius of particles by applying Stokes' formula: υ=2ρg/9μr², r=radius of cloud particle, 10⁶×μ=177.0+0.44(t+10).
The result thus obtained shows that the distribution of the masses of the cloud particles obeys the law of group distribution, as already pointed out by various authors. Besides, it is ascertained here that the group distribution may be classified into five families very distinctly, and the fact may be explained as due to the relation such that the mechanism of f_??_rmation of cloud particles varies with air mass conditions.