Hail is a form of solid precipitation consisting of ice particles, often spherical or irregular in shape. Unlike snow (ice crystals) or rain (liquid water), hailstones are composed entirely of ice, appearing transparent, opaque, or a combination.
The Atmospheric Ingredients
Hail formation relies on specific atmospheric conditions found within powerful thunderstorms, known as cumulonimbus clouds. These towering clouds extend high into the atmosphere where temperatures are below freezing. Within these clouds, strong updrafts—currents of rapidly rising air—lift water droplets and ice particles to great heights. These updrafts can reach speeds exceeding 100 miles per hour in severe storms, enabling hail growth.
Also present are supercooled water droplets at high altitudes. These liquid water droplets remain unfrozen below 32 degrees Fahrenheit (0 degrees Celsius) because they lack a nucleus, such as a dust particle or ice crystal, around which to freeze. The coexistence of strong updrafts and abundant supercooled water within a cumulonimbus cloud enables hail development.
The Hail Formation Process
A hailstone begins as a small ice particle, or embryo, forming in the upper cumulonimbus cloud. This embryo can be a frozen raindrop or a small graupel particle, a soft, opaque ice pellet. Once formed, this embryo is caught in powerful updrafts, carrying it upward through the cloud. As it ascends, it encounters supercooled water droplets.
Upon contact, the water rapidly freezes onto the surface of the ice embryo in a process called accretion. This adds a new layer of ice to the growing hailstone. The hailstone continues to be lofted upwards by the strong updrafts, collecting more supercooled water.
Eventually, it may become too heavy for the updraft to support, or it may encounter a downdraft. The hailstone then falls, potentially entering warmer cloud parts or being caught in another updraft. This cyclical motion allows the hailstone to repeatedly pass through regions rich in supercooled water. Each cycle adds new layers of ice, causing the hailstone to grow larger before it finally falls to the ground.
Factors Influencing Hail Size and Shape
Several factors determine a hailstone’s size and shape. The strength and duration of the updrafts within the thunderstorm are influential. Stronger and more persistent updrafts can suspend hailstones in the cloud for longer periods, allowing them more time to accrete additional layers of ice and grow larger. The availability of supercooled water droplets within the cloud also plays a role; a greater concentration of these droplets means more material for growth.
A hailstone’s layered appearance results from cycling through the storm’s freezing and non-freezing regions. As a hailstone moves through different parts of the cloud, it can accumulate layers of clear ice when freezing slowly in areas with more liquid water, and opaque ice when freezing rapidly in colder, drier regions. This process creates the distinct concentric layers often observed when a hailstone is cut in half. Hailstones can vary significantly in shape, from nearly spherical to irregular and lumpy, depending on its journey and collisions within the cloud.
Distinguishing Hail from Other Ice Precipitation
Hail is distinct from other forms of frozen precipitation, such as sleet, graupel, and freezing rain, due to its formation and physical characteristics. Sleet, or ice pellets, forms when snowflakes partially melt through a warm air layer, then refreeze into small pellets in a deep sub-freezing layer closer to the ground. These pellets are typically smaller and more uniform than hailstones.
Graupel, also called soft hail or snow pellets, originates when supercooled water droplets accrete onto a snowflake, forming a soft, opaque ice particle. Unlike hail, graupel is fragile and crumbles easily.
Freezing rain, conversely, occurs when precipitation falls as liquid rain through a shallow layer of sub-freezing air at the surface. The raindrops freeze upon impact with surfaces at or below freezing, creating an ice glaze.