It is the larger droplet falling through the smaller ones that grows by coalescing. Coalescence is enhanced when colliding drops have opposite electric charge. Size is also enhanced by the time a falling drop spends in a cloud. Thick clouds with strong vertical updrafts form the largest drops. A warm towering cumulus congestus because the strong updrafts increase the time spent in colliding and coalescing.
Clouds have to be present.
Heavy showers have big drops and need more time in a cloud. Thus, strong convective updrafts of the cumuliform clouds have to be there to supply this need. This partial melting of the outer fringe of the flakes is provided near the freezing temperatures on the way down to the surface. Dendrite ice crystals are the most common form of ice crystals. The maximum growth rate of ice crystals will occur when the difference in saturation vapor pressure between the water and ice is the greatest.