The coalescence process produces much of the rain that forms over the oceans and in the tropics. This process occurs as droplets fall. The larger droplets fall faster than the smaller ones. The clouds consist of droplets of supercooled water, water that is colder than the freezing point, but still liquid. But some of these droplets eventually freeze because they contain, or come into contact with, microscopic particles called freezing nuclei or ice nuclei.
Most freezing nuclei are dust particles or tiny specks of plant debris raised by the wind.
When a supercooled droplet freezes, it turns into an ice crystal. The crystal then grows by collecting water vapor. This growth produces the complicated shapes of snow crystals. As a crystal grows, it falls faster and collides with supercooled droplets, which immediately freeze onto it. This process produces soft ice pellets called graupel or small hailstones.