The importance of the form in which equation 6.30 is written is that it explicitly separates out v₁ and r₁ as both will depend on h (in the cloud).

Using 6.30 consider the case where a droplet of radius r_o is located at the base of cloud. The droplet is growing with constant collection efficiency whether its moving up or down in the cloud. When the droplet reaches the cloud base again, it now has a radius of R. Show that R is a function only of r_o and the updraft velocity w (assumed to be constant).