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Weather and Climate
How does hail form?
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1 Reply
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Hail forms within severe thunderstorms when there are strong updrafts and a specific set of conditions present. Here is a general description of how hail forms:
Updrafts: Thunderstorms have strong updrafts, which are powerful upward currents of air. These updrafts carry raindrops upward into the colder regions of the storm cloud.
Supercooling: As the raindrops are carried higher into the storm cloud, they encounter extremely cold temperatures, often below freezing. The supercooled water droplets remain in a liquid state even though they are below the freezing point.
Nucleation: The supercooled water droplets need a nucleus to freeze around. This could be a tiny ice crystal, a dust particle, or other types of condensation nuclei present in the cloud. When a supercooled droplet comes into contact with a nucleus, it freezes rapidly.
Growth: Once the supercooled droplet freezes around a nucleus, it forms a small ice pellet called a hailstone. As the hailstone descends through the cloud, it encounters supercooled water droplets, which freeze upon contact with the hailstone, causing it to grow larger.
Layers and Circulation: The hailstone is carried by the updrafts and downdrafts within the storm cloud, allowing it to accumulate additional layers of ice. This process can repeat several times as the hailstone is carried up and down within the storm.
Final Growth: The hailstone continues to grow as it moves within the storm cloud, colliding with supercooled water droplets and collecting more layers of ice. The hailstone grows larger with each collision, and stronger updrafts can suspend it longer, allowing it to accumulate more ice.
Exit from the Cloud: Eventually, the hailstone becomes too heavy for the updrafts to support, and it falls to the ground. The size of the hailstone when it reaches the surface depends on the strength of the updrafts, the amount of supercooled water available, and the time spent within the storm cloud.
Hailstones can range in size from small pellets to large, destructive stones several inches in diameter. The conditions required for hail formation are specific, which is why hailstorms are relatively infrequent compared to regular rainstorms.
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