Hail is a form of solid precipitation, distinct from snow or sleet, falling as balls or irregular lumps of ice. Hailstones vary significantly in size and structure, forming under specific atmospheric conditions, primarily within powerful storm systems.
How Hail Forms
Hail primarily develops within strong, towering cumulonimbus clouds. These clouds have immense vertical development, extending high into the atmosphere where temperatures are well below freezing. The process begins with powerful updrafts, currents of rapidly rising air within the thunderstorm. These updrafts lift water droplets and small ice crystals high into the colder regions of the cloud.
As water droplets are carried upward, they encounter below-freezing temperatures but often remain liquid, known as supercooled water. Small ice particles act as initial nuclei. Strong updrafts cause these nuclei to collide with supercooled water droplets, which instantly freeze onto their surfaces in a process called accretion. This causes the hailstone to grow.
Hailstones are repeatedly carried upward by intense updrafts, then fall back down. During each cycle, they collect more supercooled water droplets, which freeze and add new layers of ice. This cyclical journey allows hailstones to accumulate significant mass and size. Eventually, hailstones become too heavy for the updraft to support, and gravity pulls them down to the Earth’s surface.
What Makes Hailstones Different
Hailstone size is influenced by the strength and duration of thunderstorm updrafts. Stronger, more persistent updrafts suspend hailstones longer, allowing them to accumulate additional ice layers and grow larger. While many hailstones are pea-sized, some can grow to several centimeters in diameter, with recorded instances reaching sizes comparable to softballs or grapefruits.
When cut open, a hailstone reveals a distinctive “onion-like” layered structure, with alternating clear and opaque ice. These layers provide insights into the hailstone’s journey. Clear ice layers form when supercooled water freezes slowly, allowing trapped air bubbles to escape. Conversely, opaque layers form when water freezes rapidly, trapping tiny air bubbles. This alternation reflects the hailstone passing through different cloud regions with varying temperatures and supercooled water concentrations.
Hailstones are not always perfectly spherical; they can exhibit irregular shapes, lumps, or spikes. These variations result from collisions with other hailstones during their turbulent journey. Partial melting and refreezing as the hailstone descends also contribute to its final, unique form.