When does hail become very large and heavy?
Cumulonimbus clouds are very turbulent with strong up and down draughts. This gives the hail opportunities to travel several times through the supercooled water vapour, and each cycle (of accumulating water vapour on the surface and then freezing it in place) will increase the size of the hailstone. For very large hailstones to be created, the updraughts must be strong enough to carry heavy stones upwards and the arrangement of these and the downdraughts must allow many cycles of accummulation.
