Hail stones are pellets of ice that fall from the sky as precipitation, occasionally doing significant damage to property or crops, and very rarely seriously injuring or even killing people. They range in size from smaller than a pea, to the size of a grapefruit in a few recorded instances.
Severe storm warnings are issued for hailstorms when the hail stones surpass one inch in diameter. This is not just because the hail itself has the potential to do damage, but because due to the conditions necessary for hail to form, it can be a harbinger of serious thunderstorms with lightning and high winds, and even of tornado activity.
In such severe weather conditions, a convection cell forms as warm moist air rises, and cool air sinks. If the resulting clouds extend to heights of 50,000 feet or more, the moisture “super freezes,” which means that it takes the form of water rather than ice even below the normal freezing point of water. It remains in this liquid state until it comes into contact with something solid that can act as a nucleus, at which point it solidifies into ice around that object.
As moisture is being sucked up to high elevations by the storm, bits of dust, small insects, and other tiny debris can be swept upward as well. When a super frozen liquid water droplet makes contact with one of these particles, it forms a tiny hail stone of ice.
Gravity pulls these hail stones downward, as the continued upward thrust of the storm winds pushes them back up. When gravity wins, the hail stone falls to the ground. When the storm winds win, the hail stone is kept at its high altitude.
Sometimes the hail stone is able to fall part of the way toward earth, and then gets swept upwards again. As it reenters the higher altitude of super frozen water pellets, it can act as a nucleus. Just as the initial water droplet formed a hail stone when it came in contact with a piece of dust or other small particle, another water droplet now solidifies around the existing hail stone when they come in contact with each other.
Hail stones can cycle through the high altitudes multiple times like this, and grow bigger and bigger. When cut open, a hail stone is multi-layered, like an onion, or like tree rings. Eventually it grows heavy enough to overcome the storm winds and fall to the ground as precipitation.
Rachelle Oblack, “Hail.” About.com.
“What is Hail?” Wise Geek.