SEATTLE - Hail forms when rain droplets get pushed higher into the atmosphere by strong upward winds. As the droplets go higher, it freezes into an ice stone known as hail. It then becomes heavier, and falls downward, picking up another coating of raindrops on its way down.
If the updrafts are strong enough, it'll blow the hail back up again, where the coating freezes, making the hail larger, then falling back downward. This process repeats until the hail is heavier than the updraft can support, when it'll finally fall to the ground. The stronger the updraft, the larger the hail stone will become.
It's said it takes an updraft of 56 mph for a hailstone to become golf-ball sized. The largest hailstone recorded was 6" wide (weighing 1.7 pounds!) in Coffeyville, Kansas on Sept. 3, 1970.