Hail is a form of solid precipitation consisting of balls or irregular lumps of ice, distinct from the smaller, more translucent ice pellets often called sleet. These icy projectiles, known as hailstones, typically measure between 5 millimeters (0.2 inches) and 15 centimeters (6 inches) in diameter. Hailstones are products of atmospheric dynamics within towering thunderstorm clouds.
The Storm Cloud’s Role
Hail exclusively forms within thunderstorm clouds, specifically cumulonimbus clouds, which are dense, towering, and capable of impressive vertical growth. These clouds are fueled by updrafts that carry warm, moist air rapidly upward from the Earth’s surface. Updrafts can reach speeds as high as 180 kilometers per hour (110 miles per hour), suspending water droplets and ice crystals far into the atmosphere.
For hail to form, a significant portion of the cumulonimbus cloud must be below freezing. Within these frigid heights, water droplets can remain in a liquid state even when their temperature is below freezing; these are known as supercooled water droplets. Supercooled droplets, strong updrafts, and high liquid-water content are essential for hail growth.
The Journey of a Hailstone
The formation of a hailstone begins when tiny ice crystals or frozen raindrops, sometimes called graupel, are lifted into the upper, colder regions of a cumulonimbus cloud by updrafts. These initial ice particles act as nuclei, providing a surface for supercooled water droplets to freeze upon contact. This process, where liquid water freezes onto an ice particle, is called accretion.
As the hailstone grows, it moves through the turbulent cloud via updrafts and downdrafts. Each time the growing hailstone passes through a region rich in supercooled water droplets, more water freezes onto its surface, adding another layer of ice. The strength and persistence of the updrafts play a significant role in determining how long a hailstone remains suspended in the cloud and how large it can become. Hailstones continue this cycle of accretion and movement within the cloud until they become too heavy for the updrafts to support, at which point they fall to the ground.
Decoding Hailstone Layers
When a hailstone is cut open, it often reveals a characteristic internal structure resembling the concentric rings of an onion. These layers provide evidence of the hailstone’s journey and the varying conditions it encountered within the storm cloud. The alternating clear and opaque layers indicate different stages of growth and the environment in which the ice accumulated.
Opaque, milky white layers form when the hailstone collects supercooled water droplets in very cold regions of the cloud, causing them to freeze rapidly. This quick freezing traps tiny air bubbles within the ice, giving it a cloudy appearance. Conversely, clear, translucent layers develop when the hailstone passes through areas where temperatures are just below freezing, allowing supercooled water droplets to freeze more slowly. This slower freezing process permits air bubbles to escape, resulting in clearer ice.
Global Hail Hotspots
Hailstorms are not uniformly distributed across the globe; certain regions experience them more frequently due to atmospheric and geographical factors that favor severe thunderstorm development. One prominent area is the “Hail Alley” in the central United States, encompassing parts of eastern Colorado, western Nebraska, and eastern Wyoming, where hail events are notably common. This region experiences an average of seven to nine hail days each year. The convergence of dry air from the Rocky Mountains with moist air from the Gulf of Mexico creates an environment conducive to strong convection and hail formation in this area.
Other global hail hotspots exist beyond North America. Parts of Argentina, particularly the Córdoba Province, have experienced very large hailstones, with some recorded as large as 7.4 to 9.3 inches across. Regions in Australia are also prone to significant hail events. These areas often share conditions such as terrain that lifts air, the presence of frontal systems, and atmospheric instability, all of which contribute to the development of thunderstorms necessary for hail production.