Hail forms when strong thunderstorm updrafts lift liquid water high enough into subfreezing layers, causing water to freeze onto growing ice particles and create hailstones. These stones can accumulate additional layers as they move through regions of the storm with varying temperatures and liquid water content, and they fall to the ground when the updraft can no longer support their weight. Here’s a concise breakdown:
- Updrafts lift moisture: Within mature thunderheads, powerful updrafts carry raindrops upward into colder parts of the atmosphere.
- Freezing level encountered: Once lifted above the freezing level, water droplets begin to freeze, forming small ice embryos.
- Growth by accretion: As these embryos collide with supercooled liquid droplets, ice accumulates on their surfaces, causing the hailstone to grow. The rate and appearance of growth (clear vs cloudy ice) depend on how quickly freezing occurs and how much liquid water is available.
- Internal layering: Hailstones can develop concentric layers that reflect changes in temperature and moisture as they travel inside the storm, sometimes with clear or cloudy ice depending on the growth conditions.
- Updrafts and size limits: The storm’s updraft strength determines how large a hailstone can become by supporting it against gravity; when the updraft weakens or the hailstone becomes too heavy, it falls.
- Exit from the cloud: Once it leaves or reaches the edge of the storm, it may start melting depending on the ambient temperature.
Key takeaways:
- Hail is born in thunderstorm updrafts that transport water above the freezing level.
- Growth occurs through successive supercooled water droplet collisions and freezing.
- The updraft’s strength controls how big the hail can get, and the storm’s environment shapes the stone’s internal structure and appearance.
