The Basics of Hail Formation
Hail forms inside powerful thunderstorms when raindrops are carried upward by strong air currents, known as updrafts, into extremely cold areas of the atmosphere. Once the raindrops rise above the freezing level, they freeze into small ice pellets, which are the initial embryos of hailstones.
Section 2: The Growth of a Hailstone
A hailstone grows larger through a cyclical process. After freezing, the small ice pellet begins to fall but is often pushed back up by another strong updraft. As it travels through the cloud, it collides with and collects supercooled water droplets, which are water droplets that remain liquid below freezing. This collected water then freezes onto the surface of the pellet, adding a new layer of ice.
Section 3: A Practical Example
Imagine a single raindrop inside a large cumulonimbus cloud. A powerful updraft, moving at over 60 mph, lifts this drop high into the sky where temperatures are well below 32°F (0°C), causing it to freeze. The tiny ice ball falls, collecting more water, and is then caught by another updraft. This cycle repeats, adding concentric layers of ice, much like the layers of an onion, until the hailstone becomes too heavy for the updrafts to support.
Section 4: Why Hail Falls to the Ground
A hailstone finally falls to the ground when it either becomes too heavy for the thunderstorm's updraft to hold up, or when the updraft weakens. The size of the hailstone is determined by the strength and duration of the updrafts within the storm; stronger updrafts can support the growth of much larger and more destructive hailstones.