While sleet is formed by water passing through cold air and freezing on its way down to earth, hail formation is much more involved, since hail actually bounces around for awhile before finally heading for earth.
Hail is composed of balls of ice, that are formed during thunderstorms. They can range in size from small peas to softballs, and they do a lot of damage to crops, buildings, cars, and people. These icy, and sometimes destructive, balls form from tiny rain drops.
During a storm, the upper atmosphere contains cooler air, producing super cooled water droplets, that come in contact with ice crystals. Because of updrafts, these water droplets are continuously bounced back up into the cooler air, where they meet more ice crystals, and continue to grow. When they become heavy enough, they fall to the earth. In this formation theory, the size of the hailstone depends on how many times the updraft bounces the once harmless raindrop back up into the cooler air.
Another theory is that hail forms at the back, upper elevations of the storm, and is carried along, growing as it goes, until they run across a downdraft that sends them crashing to the ground.
Large hailstones, that have been traveling up and down with the updrafts for some time, develop layers of ice, so that if you cut one open, you could actually determine how many times it has been lifted back up into the cooler air. The record for one of these large stones in the U.S. is seven inches in diameter.
There are two distinct types of growth when it comes to hailstones, wet and dry. In wet growth, the air is not supercooled, and the water droplet freezes slowly. However, in dry growth, the air is supercooled and the droplets freeze immediately.
When and where hail falls is a matter of how heavy it has become, the strength of the updraft, the amount of activity inside the storm, and even the wind. When the hail becomes too heavy for the updraft, it will fall. However, if there is a lot of activity inside the storm, the hail will bounce around, much like balls in a pinball machine. This keeps them from falling. Wind can also carry them away or send them crashing down on an angle.
In the U.S., the areas most prone to hail storms are Nebraska, Wyoming, and Colorado. Because this area is in the high plains, the part of the atmosphere at 32 degrees is closer to the ground than in other parts of the country. In areas closer to sea level, hailstones have further to fall to earth, and more time to melt. Areas of hail probability, called hail swaths, can cover hundreds of square miles.
Hail may very well start out as the simple rain drop, but it definitely develops an attitude once it freezes and grows. It is one of the most damaging forms of precipitation.
http://www.nssl.noaa.gov/primer/hail/hail_basics.html
