Collision-Coalescence Process

The Collision-Coalescence Process A droplet may continue to grow by diffusion beyond 20 micrometers in diameter, however, once a droplet attains this size, growth is slow and inefficient. Droplets this large begin to collide and coalesce with other droplets as they fall through the cloud, meaning they will bump into and bond to one another and form larger drops. Updrafts in a cloud can transport a droplet upward repeatedly allowing it many opportunities to fall back down through the cloud and collide and coalesce with other droplets. Initially by diffusion, and subsequently by collision and coalescence, tiny aerosol nuclei grow into large water droplets more than 10,000 times their initial size.

Coalescence (the merging of two liquid droplets) is the only process by which precipitation-sized droplets can be formed in clouds that are warmer than 0 degrees C, and is known as the warm cloud process.