Hailstorms and Derechos

Hail forms when strong updrafts within a cumulonimbus cloud repeatedly carry small ice pellets up through layers of supercooled water droplets, which freeze onto the pellet's surface in successive layers, much like the growth rings of a tree, each pass adding another shell of ice; a hailstone finally falls to the ground once it grows heavy enough, or the updraft weakens enough, that gravity overcomes the storm's lifting force. Because sustaining a large hailstone against gravity requires an unusually strong and persistent updraft, the largest hail is almost always associated with supercell thunderstorms rather than ordinary, weaker convective cells, and a belt covering parts of eastern Colorado, Wyoming, and western Nebraska, often called "Hail Alley," experiences the highest hail frequency in North America, an effect linked to the region's high elevation and the orographic influence of the Rocky Mountains on storm development.