Hail Storming

Short answer: Hail forms inside thunderstorm updrafts. According to NOAA's National Severe Storms Laboratory (NSSL), raindrops are carried up into very cold air and freeze.

The young hailstones then grow by colliding with supercooled water drops, liquid colder than freezing, which freeze onto them. A stone grows on a seed of ice, snow or dust. Water that freezes instantly traps air bubbles and makes cloudy ice, while water that freezes slowly makes clear ice. The alternating layers reflect the changing conditions a stone passes through, not just simple up-and-down trips. Hail falls when the updraft can no longer hold up its weight, and stronger updrafts can hold bigger stones. Hail 2 inches (5 cm) or larger usually comes from supercells, storms with a rotating updraft. The largest U.S. hailstone fell in Vivian, South Dakota, on July 23, 2010: 8.0 inches across and about 1.94 lb (0.88 kg). Small hail falls at roughly 9-25 mph, while stones over 4 inches can top 100 mph. 'Hail alley', where Nebraska, Colorado and Wyoming meet, averages seven to nine hail days a year. Insurers count hail as a major part of the tens of billions of dollars in U.S. severe-storm losses each year. The footage shows hail falling past a porch and windows.