Hail causes billions of dollars in damage each year. Hail-producing thunderstorms can occur in every state; however, there is a region of the United States called “Hail Alley” where hail occurs most frequently, seven to nine days per year. These states are Wyoming, Colorado and Nebraska.
How does hail form? Hailstones grow by colliding with super-cooled (liquid water that can exist at temperatures as low as -40°F) water drops. This super-cooled water will freeze on contact with ice crystals, frozen raindrops or dust. Thunderstorms that have strong updrafts, around 100 mph, keep lifting the hailstone up to the top of the cloud where they encounter more super-cooled water and continue to grow.
The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft, the larger the hailstone can grow. Most hailstones are the size of peas, dimes and pennies, one-fourth an inch to half an inch in diameter. Some hail can reach the size of a baseball, 2 3/4 inches.
In the strongest thunderstorms, hail can grow much larger. Hail can reach the size of a grapefruit, which is 4 inches in diameter. The largest hailstone ever recorded hit the ground on June 23, 2010 in Vivian, South Dakota: 8 inches in diameter!