Hail is a frozen form of precipitation generally associated with severe thunderstorms, especially those which form in advance of cold fronts. Large hailstones is also seen as an early indicator of an approaching tornadic storm cell, and they can cause significant damage to exposed structures and vehicles. There are also documented cases where people have been seriously injured or even killed as a result of a heavy hailstorm.
In order for most examples of hail to form, a number of meteorological conditions need to be in place first. Moisture-laden storm clouds must build up to heights approaching 50,000 feet or more. The air temperature at those heights is cold enough to super freeze water droplets, which means they remain liquid in form but below freezing in temperature. If those super frozen droplets made contact with anything solid, called a nuclei in meteorological terms, they would freeze immediately.
The problem is that there aren't too many solid objects floating around at 50,000 feet above the ground naturally. When the winds inside a storm cloud begin to circulate, a powerful updraft is created. This updraft can pull dust particles, insects or tiny ice crystals up to the cloud's highest levels. When these natural nuclei come in contact with the super frozen droplets, they form a small ball of solid ice. Eventually these new ice formations fall towards the ground, but they could be swept back into the top of the cloud formation over and over again by the updrafts. A new layer of frozen water droplets is added to the young hailstone during each trip.
Eventually the ice droplets become too heavy to be supported by the storm's updrafts. The hailstones then fall to the ground like rain or sleet or snow. The size of an individual stone depends on how many times it was drawn back into the upper levels of the storm cloud before it finally fell. If a large piece of hail were cut in half, there would be a number of layers of ice, much like an onion's layers. Stones may be as small as a pea, or under extreme circumstances could become the size of a softball or even grapefruit.
Not all storm clouds produce hail, since the updrafts may not be strong enough to force nuclei materials into the upper levels or the frozen precipitation may melt before it reaches the ground. A tornadic super cell, on the other hand, can have a very powerful updraft capable of picking up dust particles and other nuclei for hailstones. This is why a strong hailstorm is often an indicator that a tornado is approaching or at least about to form.