Hail is a dramatic form of precipitation resulting from extreme atmospheric forces at work within a severe thunderstorm. Understanding hail requires looking high up into the towering clouds where intense vertical motion and supercooled water droplets combine to create solid chunks of ice.
Defining Hail and Its Physical Structure
Hail is defined as a form of solid precipitation consisting of lumps of ice, formally classified as hail only if the individual stone measures 5 millimeters (about 0.2 inches) or larger in diameter. Unlike other forms of ice, hailstones are hard, dense, and often reveal a complex internal structure when cut open. This structure is characterized by alternating, concentric rings of clear and opaque ice, similar in appearance to the rings of a tree. The number of these distinct layers indicates the minimum number of growth cycles the hailstone experienced within the storm cloud.
The Internal Mechanics of Hail Formation
Hailstones begin their life as a tiny nucleus, often a frozen raindrop or a small piece of soft ice called graupel, suspended within a massive cumulonimbus cloud. Growth is dependent on a strong, sustained updraft—a powerful column of rising air that prevents the particle from falling. This updraft repeatedly carries the ice particle high above the freezing level into regions filled with supercooled water droplets, which remain liquid even at temperatures below \(0^\circ\)C. As the hail embryo collides with these droplets, they freeze instantly onto its surface, a process called accretion.
The appearance of the resulting ice layer depends on the temperature and the concentration of water droplets it encounters. When the hailstone is in a very cold, dry region, the droplets freeze instantly, trapping tiny air bubbles and creating a milky white, opaque layer, known as dry growth. Conversely, if the hailstone passes through a warmer section just below freezing, the water spreads across the surface before slowly freezing, forming a clear, dense layer through wet growth. The hailstone continues this cycle until its weight finally exceeds the force of the updraft, causing it to plummet to the ground.
Atmospheric Conditions Required for Hail
The formation of hail requires a specific environment capable of generating the immense power of a severe thunderstorm. A primary requirement is high atmospheric instability, quantified by Convective Available Potential Energy (CAPE), which provides the fuel for the storm’s powerful vertical motion. High CAPE values, often exceeding 2000 Joules per kilogram, are necessary to generate the intense updrafts capable of suspending large hailstones. The storm also needs a deep layer of cold air aloft, meaning the cloud must extend to great heights where temperatures are far below freezing.
Crucially, the freezing level—the altitude at which air temperature reaches \(0^\circ\)C—must be relatively low to the ground. If the freezing level is too high, the hailstone will melt completely during its long descent through the warmer air below the cloud base. Furthermore, dry air in the mid-levels of the atmosphere is beneficial, as it promotes evaporational cooling that can lower the freezing level, increasing the hailstone’s chance of survival to the surface.
How Hail Differs from Sleet and Graupel
Hail is frequently confused with other types of frozen precipitation, but its formation and physical characteristics set it apart. Sleet, formally known as ice pellets, forms when snowflakes melt completely into raindrops as they fall through a layer of warm air, then refreeze into small, transparent pellets upon entering a thick layer of sub-freezing air near the ground. This process requires a specific temperature inversion that hail does not. Graupel, often called soft hail or snow pellets, is a separate phenomenon that forms when supercooled water droplets freeze onto a snow crystal, creating a small, opaque, and easily crushable pellet. Unlike hail, graupel is soft, has a low density, and is typically much smaller, measuring less than 5 millimeters.