Hail is a type of solid precipitation that falls as balls or irregular lumps of ice, known as hailstones. Its formation involves a complex interplay of atmospheric conditions within powerful thunderstorms.
Essential Atmospheric Conditions
The formation of hailstones begins with specific atmospheric conditions that support the development of intense thunderstorms. A significant factor is strong updrafts, powerful currents of air rising rapidly from the Earth’s surface. These updrafts are responsible for lifting water droplets and ice particles high into the atmosphere, suspending them within towering cumulonimbus clouds.
Another condition is the existence of cold temperatures aloft, at altitudes where the air temperature is well below freezing. At these frigid heights, water droplets can remain in a liquid state, known as supercooled water. Ample moisture in the atmosphere is also necessary, providing the abundant supply of water droplets that are the fundamental building blocks for hail formation. This moisture contributes to the high liquid water content needed for hail growth.
The Hailstone Formation Process
The journey of a hailstone begins with initial ice nuclei, often small ice crystals or frozen raindrops. These tiny particles serve as the foundation upon which the hailstone will grow. As strong updrafts carry these embryos upward into regions of the cloud where supercooled water droplets are prevalent, the process of accretion begins.
Accretion involves these supercooled water droplets colliding with and instantly freezing onto the surface of the ice embryo. This continuous collection causes the hailstone to grow. The hailstone then cycles through different parts of the storm, carried repeatedly upward by strong updrafts, accumulating more layers of ice.
This cycling, combined with temperature and water content changes, leads to the characteristic layered structure observed when a hailstone is cut open. These layers can alternate between clear and opaque ice, reflecting how quickly the water froze. This layered appearance provides a record of the hailstone’s journey through the thunderstorm’s thermal environments.
Why Hailstones Vary in Size and Shape
The final size of a hailstone is largely influenced by the strength and duration of the updrafts within the thunderstorm. Stronger updrafts can suspend hailstones for longer periods, allowing more time for them to collect supercooled water and grow to larger dimensions. For instance, pea-sized hail requires updrafts of about 24 miles per hour, while grapefruit-sized hail can necessitate updrafts approaching 100 miles per hour. The more times a hailstone is cycled through the updraft and downdraft regions of the cloud, the greater its potential to accumulate additional ice layers and increase in size.
The availability of supercooled water droplets also significantly impacts the growth rate and ultimate size of hailstones. A higher concentration of these droplets means more material is available for the hailstone to accrete during its passage through the cloud. Furthermore, the frequency of collisions with other ice particles and water droplets, along with the amount of melting that occurs during its descent, affects the hailstone’s final size and shape. Hailstones can be spherical, conical, or irregular, with their shape influenced by collisions and partial melting and refreezing.