Collision / Coalescence
In many parts of the world the air is too warm for ice crystals to form. This being the case, rain and snow cannot develop following the Bergeron Process. Instead, tiny droplets form as they collide into one another creating larger and larger droplets. The coalescence of droplets into larger droplets can only take place if the droplets have an opposite electrical charge. That is to say that if one droplet has a positive charge and the other a negative charge, then they will coalesce (combine) upon collision. Otherwise they will just bounce off of one another.