Hail

Hail is a form of frozen precipitation that's created by strong thunderstorms with fast updrafts—air being pulled upward into a thunderstorm. It can cause serious damage, especially to cars, aircraft, glass-roofed structures, and most notably, farmers' crops. Hail causes approximately $1 billion in property and crop damage each year. The costliest hailstorm happened in April 2001, from eastern Kansas to southwest Illinois, including the St. Louis area. Property damage in this storm exceeded $2.4 billion in 2010 dollars. Deaths from hail are rare—the last known death caused by hail in the U.S. was in the year 2000, when a man was killed by softball size hail in Fort Worth, Texas.

Hail Formation

Figure 1. An idealized graphical explanation of how hail forms.

Hail is formed when very strong thunderstorm updrafts meet supercooled water droplets. Supercooled water droplets are liquid water drops that are surrounded by air that is below freezing, and they're a common occurrence in thunderstorms. There are two methods of hail stone formation and growth that give hail stones their "layered" look.
Wet Growth
A tiny ice crystal will be the nucleus of the hail stone. In wet growth, supercooled water droplets collide with and spreads across the ice nucleus. Since this process is relatively slow (slower than the dry growth process) it results in a layer of clear ice.
Dry Growth
Unlike wet growth, the supercooled water meets the ice nucleus and immediately freezes. Because this process is so fast, everything within the supercooled droplet, including small air bubbles, freezes into the layer, which gives it a cloudy look.
Rain and hail is what creates the "bounded" portion of the bounded weak echo region (BWER) on radar. The weak echo region is created by a strong updraft, which also helps the hail to grow.

1. Hail forms and is carried upward through the storm by the updraft and held above the freezing line
2. The hail stone collides with supercooled droplets and grows in size
3. When the hail becomes too large and heavy to be supported by the updraft, it falls to the ground

Record-Setting Hail

The largest officially recognized hailstone on record to have been "captured" in the U.S. was that which fell near Vivian, South Dakota in 2010. It measured 8.0 inches in diameter, 18.5 inches in circumference, and weighed in at 1.9375 pounds. Mr. Lee Scott, who collected the monster stone, originally planned to make daiquiris out of the hailstone but fortunately thought better and placed it in a freezer before turning it over to the National Weather Service for certification.
More on record-setting hail from our Weather Historian, Christopher C. Burt.

Figure 2. The largest hail stone on record in the U.S. was an eight-inch monster that fell on July 23, 2010 in Vivian, South Dakota.

The following video was shot in an incredible hail storm in Oklahoma in 2010. It's not often that you get to see softball-size hail fall into a swimming pool from 30,000 feet. The video really gets good around 1 minute in.



http://www.wunderground.com/resources/severe/hail.asp