Hail is a form of solid precipitation consisting of ice balls or irregular lumps, known as hailstones. This natural phenomenon forms exclusively within specific atmospheric conditions. Understanding how these icy pellets develop and vary in size reveals the powerful dynamics at play within our atmosphere.
The Atmospheric Ingredients
Hail forms within strong thunderstorms, specifically within towering cumulonimbus clouds. These clouds are characterized by significant vertical development, reaching heights where temperatures are well below freezing. The presence of robust updrafts, which are powerful upward currents of air, is fundamental for hail formation. These updrafts can be incredibly strong, sometimes exceeding 100 mph.
Another essential component is supercooled water droplets. These are liquid water droplets that remain unfrozen even at temperatures below 0°C (32°F). Supercooled water will freeze instantly upon contact with an ice particle or other freezing nucleus. Without strong updrafts to lift water high into freezing regions and the presence of supercooled water, hailstones cannot begin their growth process.
The Hailstone’s Icy Ascent and Descent
The formation of a hailstone begins with a small ice embryo, often a frozen raindrop or a graupel pellet. This tiny ice particle is then caught by the powerful updrafts within the cumulonimbus cloud, which propel it upward into the colder regions. As the embryo ascends, it collides with numerous supercooled water droplets. These droplets freeze instantly upon impact, adding layers of ice to the growing hailstone, a process known as accretion.
The hailstone continues to grow as it is carried higher into the cloud. However, as it gains mass, it may become too heavy for the updraft to support, causing it to fall into warmer parts of the cloud. Before it can melt completely, it can be caught by another strong updraft and lifted back into the freezing zone. This cyclical journey of ascent and descent, through varying temperature and moisture regions within the cloud, allows the hailstone to accumulate multiple layers of ice.
When a hailstone is cut open, it often reveals a layered structure, similar to the rings of an onion. These layers can alternate between clear and opaque ice, indicating different growth conditions. Clear layers form when the hailstone passes through regions with abundant supercooled water, allowing the water to spread and freeze slowly, releasing trapped air. Opaque, milky layers form when the water freezes rapidly upon impact in colder, drier regions, trapping air bubbles.
Why Hailstones Vary in Size
The final size of a hailstone is determined by the strength and duration of the updrafts within the thunderstorm. Stronger updrafts can suspend hailstones for longer periods, allowing them to undergo more growth cycles and accumulate additional layers of ice. For instance, a pea-sized hailstone requires an updraft of about 24 mph, while a grapefruit-sized hailstone needs updrafts nearing 100 mph to remain aloft. This extended suspension directly correlates with larger hailstone development.
The amount of supercooled water available in the cloud also influences hailstone size. More liquid water provides more material for the hailstone to accrete during its journey. The height a hailstone reaches within the cloud and the number of times it cycles through the updraft and downdraft zones contribute to its ultimate dimensions. These factors collectively dictate whether a hailstone remains small or grows larger before gravity overcomes the updraft and it falls to the ground.