Hail forms when strong convective thunderstorms contain powerful updrafts capable of carrying water droplets and ice particles upward into very cold parts of the cloud, where they freeze and grow. Hailstones generally develop inside cumulonimbus clouds, especially where strong vertical motion provides repeated opportunities for growth. Liquid water can remain supercooled below zero degrees Celsius, meaning it stays liquid despite being below its normal freezing point.
When supercooled droplets collide with an ice particle, they can freeze onto its surface, a process called accretion or riming. If the growing particle is repeatedly transported through regions containing abundant supercooled liquid water, it can accumulate multiple layers of ice and become a substantial hailstone. The central requirement is therefore not simply cold air but a combination of intense convective updrafts, abundant moisture, supercooled liquid water and suitable temperatures within a deep thunderstorm.
Strong updrafts are crucial because they allow developing hailstones to remain suspended or repeatedly rise through regions favourable for growth. A particle that falls rapidly out of a weak cloud has limited time to accumulate water and ice, whereas a powerful thunderstorm can transport particles upward at speeds comparable to or greater than their fall speeds. During these cycles, hailstones encounter supercooled droplets that freeze upon impact.
Depending on the rate at which water freezes and the temperature and liquid water content of the surrounding cloud, the resulting ice can be relatively clear, opaque or layered. Rapid freezing can trap air bubbles and create milky ice, while slower freezing can produce clearer layers. Hailstones can therefore preserve evidence of changing conditions within the thunderstorm as successive layers accumulate. Large hail generally requires particularly strong and sustained updrafts, although the exact relationship depends on storm structure and microphysical conditions.
Hail growth is closely connected to the complex internal structure of thunderstorms. A mature severe thunderstorm can contain an updraft region, downdraft regions, precipitation shafts and strong horizontal airflow. Hail embryos may originate from small ice particles, graupel or frozen droplets and can be transported repeatedly through the updraft. Some storms have rotating structures that organise the updraft and allow it to persist for long periods, providing favourable conditions for large hail development.
The presence of a strong updraft does not automatically guarantee hail because the cloud must also contain sufficient supercooled liquid water and appropriate thermodynamic conditions. Similarly, large amounts of precipitation do not necessarily indicate large hail. Hail production emerges from the interaction of storm dynamics, temperature, moisture, particle trajectories and cloud microphysics.
Hailstones can exhibit layered internal structures because their growth conditions change as they travel through different parts of the cloud. A wet growth process occurs when abundant supercooled liquid water reaches an ice surface faster than it can freeze immediately, allowing a film of liquid water to exist temporarily before freezing. This can produce relatively clear ice. Dry growth occurs when droplets freeze rapidly after impact, trapping bubbles and producing more opaque ice.
A single hailstone can alternate between these regimes, producing visible layers when cut open. Hailstones can also collide with other ice particles, break apart or fragment, creating new hail embryos. The final size and structure therefore record a complicated history of repeated ascent, descent, collision, freezing and melting. Researchers can study hailstone structure to infer aspects of the storm environment in which it formed.
Hail is different from snow, sleet and ordinary ice because its formation mechanism is dominated by strong convective cloud dynamics and repeated accretion of supercooled liquid water. Snow develops primarily through deposition and aggregation of ice crystals, often in stratiform or less violently convective clouds. Sleet or ice pellets are relatively small frozen precipitation particles produced when melted precipitation refreezes before reaching the surface. Hail, by contrast, can become centimetres in diameter and sometimes larger under exceptional conditions.
Hailstones are typically hard masses of ice with irregular or rounded shapes, although their exact geometry depends on growth and collision history. The distinction is important because a brief period of small ice pellets is not equivalent meteorologically or physically to a severe hailstorm. Hail also tends to be associated with thunderstorms and can occur even when surrounding conditions at the surface are relatively warm.
The size of hail depends strongly on the strength and persistence of the storm updraft, the amount and distribution of supercooled liquid water, the duration of growth and the particle trajectory through the cloud. Large hailstones require enough mass to survive their descent and are more likely when powerful updrafts keep them aloft long enough to accumulate substantial ice. Once a hailstone becomes too heavy for the updraft to support, it begins descending towards the surface.
It may continue growing during descent if it encounters suitable liquid water or other hydrometeors, although melting can also reduce its size in warm air below the cloud. Hailstones can fracture or partially melt before reaching the ground. The largest observed stones therefore represent the outcome of unusual combinations of storm dynamics and microphysics rather than a simple function of surface temperature or rainfall rate.
Hail can cause substantial damage because its concentrated mass and high fall speed allow it to deliver considerable kinetic energy. Agricultural crops are particularly vulnerable because leaves, stems, flowers and fruit can be damaged or destroyed. Vehicles, windows, roofs, solar panels and other exposed structures can also be damaged by large hail. Aviation is another concern because aircraft encountering severe hail can experience damage to windshields, engines and other components.
Weather radar is widely used to identify thunderstorms with characteristics associated with hail, while specialised algorithms use radar reflectivity, dual polarisation measurements and environmental data to estimate hail potential. Hail pads and automated sensors can also provide ground observations. Forecasting remains probabilistic because hail depends on small scale storm processes that are difficult to observe completely.
A common misconception is that hail is simply frozen rain. Although both consist of water in solid form at some stage, hail usually develops through repeated growth inside strong thunderstorms rather than by ordinary raindrops simply freezing while falling. Another misconception is that hail can only occur during very cold weather. Hail forms high inside thunderstorms where temperatures are low, but it can fall to the ground through warm air and therefore occur during summer thunderstorms.
Hail also does not require a long period of freezing at the surface because its solid form is maintained during descent by its size and the relatively short duration of exposure to warm air. The scientific explanation of hail therefore requires understanding convective instability, thunderstorm updrafts, supercooled liquid water, ice accretion and hydrometeor trajectories rather than treating hail as a simple surface freezing phenomenon.
