Hail is a form of precipitation consisting of irregular lumps of ice that only develops within a cumulonimbus cloud, also known as a thunderstorm cloud. This specific cloud type possesses a unique combination of great vertical height, frigid temperatures, and strong internal wind currents necessary for hail production. The atmospheric requirements for creating a hailstone—lifting liquid water far above the freezing level and holding it there long enough to grow—are not met by any other cloud type.
Vertical Extent and Necessary Temperature Gradients
The foundation for hail formation begins with the large physical scale of the cumulonimbus cloud, which can reach altitudes of up to 60,000 feet (18 kilometers), extending near the boundary of the stratosphere. This vertical stretch is a prerequisite because it ensures the cloud mass extends significantly above the \(0^{\circ} \text{C}\) isotherm, the altitude where temperatures drop below freezing. The sheer height of a cumulonimbus guarantees vast internal regions where the air temperature falls far below zero.
Within these high-altitude regions, supercooled water is abundant. This is liquid water that remains unfrozen even at temperatures well below \(0^{\circ} \text{C}\), sometimes reaching \(-40^{\circ} \text{C}\). This liquid water freezes instantly upon contact with an ice crystal or a tiny nucleus. The wide range of sub-zero temperatures within the cloud’s structure establishes the thermal gradient required for the subsequent growth phases of a hailstone.
The Role of Strong Updrafts and Supercooled Water
The dynamic force needed to sustain hail is provided by the strong, rapidly rising air currents, or updrafts, found only in cumulonimbus clouds. These updrafts are generated by the instability of a thunderstorm, which lifts warm, moist air from the ground into the upper atmosphere. The velocity of these vertical winds is exceptional, often exceeding 50 miles per hour, and can surpass 100 miles per hour in storms that produce large hail.
This upward force counteracts gravity, holding nascent ice particles and water droplets aloft for extended periods within the cloud. As the updraft carries liquid water droplets high above the freezing level, they become supercooled. These suspended droplets and ice crystals collide, initiating the growth process. This growth is only possible because the updraft prevents the particles from falling prematurely, allowing for the accumulation of more mass.
Accretion and the Hailstone Growth Process
Hailstones begin as small ice pellets or frozen raindrops, often called graupel, held within the cloud’s updraft. The actual growth occurs through accretion, where the ice particle sweeps up and collides with the dense population of supercooled water droplets. Each collision causes the liquid droplet to freeze onto the surface of the particle, adding a new layer.
The hailstone often cycles vertically through different regions of the cloud, which creates its characteristic layered structure. When the stone passes through a zone where temperatures are very cold, supercooled droplets freeze instantly upon contact, trapping air bubbles and forming a milky, opaque layer of ice (dry growth). Conversely, in slightly warmer, sub-freezing parts of the cloud, the water freezes slowly, allowing air bubbles to escape and resulting in a clear, dense layer of ice (wet growth). The stone continues to gain mass until its weight overcomes the lifting capacity of the cumulonimbus updraft, causing it to fall to the ground.