Basically, a droplet of water falls, freezes, and is blown back up over and over and over again without hitting the ground. Each time it keeps accumulating more water droplets that keep freezing to the growing hail stone. That's how all hail is formed. When the hail stone is too big for the winds to keep blowing it back up again, it falls. The stronger the updrafts, the bigger the hail will get before falling to the ground. That's why it takes a pretty powerful storm to make a big hail stone -- the winds have to be strong enough to blow an almost baseball-sized piece of ice back upward again for it to keep growing.
