Hail is precipitation composed of solid ice, distinct from smaller, softer ice pellets like sleet or graupel. While most hailstones are small, a rare few grow into dense chunks the size of baseballs or larger. This massive growth requires extreme atmospheric conditions, transforming an ordinary thunderstorm into an efficient ice-producing factory. Stones exceeding three inches in diameter have the potential to cause widespread damage to property and agriculture.
The Required Environment: Supercell Thunderstorms
The largest hailstones are almost exclusively produced by supercell thunderstorms. A supercell is defined by the presence of a mesocyclone, a deep, persistent, and rotating column of rising air. This rotation is generated by wind shear, where wind speed and direction change significantly with altitude.
This spinning motion is crucial because it separates the storm’s powerful updraft (rising air) from its downdraft (falling precipitation). Keeping these forces apart prevents the supercell from collapsing under the weight of its own rain and hail. This longevity creates the stable environment necessary to grow ice particles to extreme dimensions.
The Mechanism of Hailstone Growth
The hailstone’s journey begins high in the cloud where temperatures are below freezing. A small particle, often a frozen raindrop or a soft ice pellet called graupel, acts as a seed or “hail embryo.” This embryo is immediately swept into the storm’s powerful updraft.
As the embryo travels upward, it collides with supercooled water droplets, which are liquid water existing below the normal freezing point. The embryo collects and freezes these droplets through a process called accretion, the primary mechanism of hail growth. Each collision adds a new layer of ice to the stone, steadily increasing its mass and density.
Achieving Extreme Size Through Updraft Cycling
Achieving baseball-sized hail requires an updraft of exceptional strength that can keep a heavy stone suspended for a prolonged time. A typical baseball-sized hailstone requires an updraft speed of approximately 75 to 80 miles per hour just to keep it from falling. The hailstone continues to grow as long as the updraft velocity equals or exceeds its terminal velocity.
To reach massive sizes, the hailstone must spend extended time traveling through the storm’s richest moisture zones. This movement is not a simple up-and-down cycle but a complex, spiraling path along the periphery of the rotating updraft. This path repeatedly exposes the stone to fresh supercooled water, maximizing the time it has to accumulate mass.
Decoding the Hailstone: Layers and Structure
When a large hailstone is cut open, its internal structure reveals an onion-like pattern of alternating layers, acting as a frozen record of its journey. These layers are defined by two distinct growth regimes: wet growth and dry growth.
Wet growth occurs in slightly warmer cloud regions where supercooled water freezes slowly. This allows trapped air bubbles to escape, resulting in a layer of clear, dense ice. Conversely, dry growth happens in much colder regions where water droplets freeze instantly upon impact. This rapid freezing traps tiny air bubbles, creating an opaque, milky-white layer. Scientists use the sequence of these rings to reconstruct the hailstone’s path through different temperature and liquid water content zones before it finally fell to the ground.