What is hail? How it is formed?

The hard pellets of ice having diameter of 5 mm or more that fall from Cumulonimbus clouds, and are generally associated with thunderstorms is known as hail.

They are formed by the rapid ascent of moist air; the water drops freeze, and the size of the pellets increases as more water Vapour freezes on to their surface. When the pellets are heavy enough to overcome the resistance of the ascending air currents, they drop during their downward journey, they may still grow by assuming fresh layers of ice from the super cooled water drops in the moist air.