Hail is not simply frozen raindrops. Hail forms when a water
droplet is carried into the upper part of a storm, freezes, falls,
collects a new layer of moisture, then rises again to refreeze,
this time into a larger piece of ice. This cycle repeats until the
ahilstone is to heavy to be held up by air currents.
Large hail occurs in thunderstorms with very strong updrafts
that can keep such large objects suspended. Typically the storm is
a supercell, a thunderstorm with a powerful, rotating updraft
capable of producing large hail and strong tornadoes.