Hail forms on condensation nuclei such as dust, bugs, or ice crystals, when super cooled water freezes on make contact with. In clouds contains large numbers of super cooled water droplets, these ice nuclei grow quickly at the expense of the liquid droplets because the saturation vapor pressure over ice is slightly less than the saturation vapor pressure over water. If the hail stones grow large enough, latent heat released by further freezing may melt the external shell of the hail stone. The development that follows, usually called wet growth, is more efficient because the liquid outer shell allows the stone to accrete other smaller hail stones in addition to super cooled droplets.
Once a hailstone become too heavy to be supported by the storm's updraft it falls out of the cloud. The reason rain can't fall, is typically because of the tough winds inside a thunderstorm cloud. These winds hold the rain and freeze it. As the process repeats, the hail grows gradually larger. When a hail stone is cut in half, a series of concentric rings, like that of an onion, are revealed. From these rings we can determine the total number of times the hail stone had traveled to the top of the storm before falling to the ground.