How Hail Grows Extreme

Reading a Hailstone in Hand—and Under Polarized Light

Hold a newly fallen 12-centimeter hailstone beside a millimeter scale—never a coin or golf ball alone, whose perspective shifts with camera distance—and two details stand out: radiating finger-like icicle lobes and concentric internal rings of milky and transparent ice. For decades, popular science diagrams explained these rings by claiming hailstones fall out of a storm, get sucked back up to freeze clear water, and repeat five or six vertical cycles.

Trajectory modeling and isotopic analysis led by researchers at the NOAA National Severe Storms Laboratory (NSSL), NCAR, and the Insurance Institute for Business & Home Safety (IBHS) demonstrate a far more coherent physical path. Inside a rotating supercell mesocyclone, updrafts are tilted and wide—often 8 to 15 kilometers across. Graupel or frozen raindrop embryos enter along the updraft periphery and glide horizontally across the vault between −10 °C and −30 °C where upward wind speed closely balances the growing stone's terminal fall speed.

• Schumann–Ludlam Limit (Wet vs. Dry Shells)

  Every gram of supercooled liquid water freezing onto a hailstone releases 334 joules of latent heat. When liquid water concentration is modest or air is very cold, heat conducts away quickly: droplets freeze instantaneously on impact, trapping tiny air bubbles as opaque white ice (dry growth). Entering a liquid-rich core pushes surface temperature to 0 °C, leaving a thin water skin that freezes slowly without air bubbles into transparent ice (wet growth)—creating rings during a single horizontal pass.

• Why Giant Stones Grow Spiky Lobes

  Tumbling through wet-growth regions, meltwater flows hydrodynamically toward low-pressure zones on the stone's wake and equatorial edges, forming icicle-like cusps that capture even more supercooled droplets by increasing surface area without adding proportional density.