Hail

Hail is a formidable form of solid precipitation, distinct from other icy phenomena due to its unique formation process and potential for significant impact. While seemingly simple ice chunks, hailstones are the product of powerful atmospheric dynamics within towering thunderstorm clouds. Understanding hail involves delving into the intricate dance of moisture, temperature, and strong air currents.

What causes hail?

Hail is fundamentally caused by robust updrafts within large, convective clouds known as cumulonimbus clouds (thunderstorm clouds). These updrafts are powerful currents of air rising rapidly from the Earth's surface, driven by intense heating and atmospheric instability.

The process begins when water droplets are carried high into the atmosphere by these updrafts, reaching altitudes where temperatures are well below freezing (0C or 32F). At these frigid heights, the water droplets become supercooled, meaning they remain in a liquid state even though their temperature is below their freezing point. When these supercooled droplets collide with a tiny ice crystal or other condensation nuclei, they instantly freeze, forming a small ice embryo.

This ice embryo is then repeatedly lifted and dropped within the turbulent updraft and downdraft regions of the thunderstorm. As it travels, it collides with more supercooled water droplets, which freeze onto its surface. This process of accumulation, known as accretion, causes the hailstone to grow in size. The hailstone continues to cycle through the cloud, gaining layers of ice with each pass, until it becomes too heavy for the updraft to support, at which point it falls to the ground.

What determines the size of hail?

The size of a hailstone is primarily determined by several key factors:

• Strength and persistence of updrafts: The stronger and more sustained the updraft, the longer a hailstone can remain suspended within the cloud, allowing more time for it to gather supercooled water and grow larger. Supercell thunderstorms, with their exceptionally powerful and rotating updrafts, are particularly prone to producing very large hail.
• Amount of supercooled water: A rich supply of supercooled water droplets within the cloud is crucial for rapid hailstone growth. The more liquid water available for accretion, the faster the hailstone can increase in size.
• Vertical extent of the thunderstorm: Taller thunderstorms offer a greater vertical distance for hailstones to travel and grow.
• Freezing level height: A lower freezing level (the altitude at which the temperature drops to 0C) provides a larger volume of the cloud where supercooled water exists, increasing the opportunity for hailstones to grow.
• Time spent in the cloud: The longer a hailstone remains aloft within the optimal growth region of the thunderstorm, the larger it can become.

Hailstones are typically measured by comparing them to common objects (e.g., pea-sized, golf-ball sized, softball-sized). Hail measuring 2.5 cm (1 inch) or larger is generally considered "severe" and capable of causing significant damage.

The layered structure of hailstones

If you slice open a large hailstone, you often observe concentric layers, much like the rings of an onion. These layers can vary in appearance, alternating between clear (transparent) and opaque (milky white) ice. This layered structure provides clues about the hailstone's journey within the thunderstorm:

• Clear layers form when the hailstone passes through a region of the cloud with a high concentration of supercooled water droplets where temperatures are just below freezing. In these conditions, the water freezes slowly, allowing trapped air bubbles to escape, resulting in clear ice.
• Opaque layers form when the hailstone encounters colder regions of the cloud with fewer supercooled water droplets. Here, the water freezes rapidly upon impact, trapping tiny air bubbles within the ice, which gives it a milky, opaque appearance.

The number and thickness of these layers indicate how many times the hailstone was lifted and carried through different temperature and moisture regimes within the thunderstorm before finally falling.

What's the difference between hail and snow?

While both hail and snow are forms of frozen precipitation, their formation processes, characteristics, and typical conditions for occurrence are distinct.

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In essence, hail is not just frozen rain—it's the product of a turbulent atmosphere where water, wind, and temperature interact in a delicate yet powerful balance. From its layered structure to its destructive potential, hail offers a fascinating glimpse into the inner workings of severe thunderstorms.