Believe it or not, the origin of ice and snow particles in clouds remains a bit of a mystery. Sure, we know that by cooling water droplets, some of them are going to freeze, due to the presence of dust particles or other ice-forming nuclei. So people have proposed various multiplication mechanisms such as the freeze-shattering of droplets. In this scenario, a shell of ice that forms around a liquid droplet would shatter as the water inside freezes and expands. A variant on this, known as the Hallett-Mossop Mechanism , consists of water drops shattering as they collide with and freeze on hail ice particles.
Some clouds seem to have very sharp edges, while others have diffuse, wispy outlines.
In essence, a cloud is a region in which the air is supersaturated with water vapor and in which ice or water particles can nucleate. They are often dynamic in that the air currents cycle in and out of clouds. The edge is the point at which the air is undersaturated, so that the particles begin to evaporate. Since water is that much more volatile than ice, water clouds tend to have very sharp edges, whereas ice particles can survive for much longer in undersaturated regions - hence the wispy outlines. Another phenomenon seen in some ice clouds is the halo.