COLLISION AND COALESCENCE

Because vapor deposition is less effective for larger drops, collision and coalescence is the main process for forming warm raindrops greater than 12 to 20 microns. Large rain droplets (the collector drops) overtake and collide with smaller droplets (the collected drops) owing to their greater terminal fall velocity. If the droplets are charged or an electric field exists (which commonly does in natural clouds), the droplets will nearly always coalesce (stick together to for a single larger drop) if they collide.