How Does Hail Form
Inside of a thunderstorm are strong updrafts of warm air and downdrafts of cold air.
If a water droplet is picked up by the updrafts...it can be carried well above the freezing level. With temperatures below 32F...our water droplet freezes.
As the frozen droplet begins to fall...carried by cold downdrafts...it may thaw as it moves into warmer air toward the bottom of the thunderstorm
But...our little half-frozen droplet may also get picked up again by another updraft...carrying it back into very cold air and re-freezing it. With each trip above and below the freezing level our frozen droplet adds another layer of ice.
Finally...our frozen water droplet...with many layers of ice - much like the rings in a tree...falls to the ground - as hail!
National Weather Service