Brief Introduction to the Formation of Hail

Tiny water droplets condense on dust particles inside clouds, turn into ice crystals, and start falling to the ground.

Due to strong updrafts inside the cloud they are blown back up. Further water droplets attach themselves and freeze as well.

This process repeats itself until the ice crystals are sufficiently heavy so they will be no more caught by the updraft and thus fall from the cloud as hail stones.

Possible extreme consequences of that you can view in the following: