Hail is a type of precipitation made of solid ice that forms inside powerful thunderstorms. Unlike snow, which develops when water vapor freezes into ice crystals, hail grows when raindrops or other liquid water freeze and accumulate on an ice particle carried through a storm by strong rising air currents. As the ice grows, the hailstone can travel through different parts of the storm, collecting additional layers of ice until it becomes too heavy for the rising air to support it. It then falls to the ground.
Hailstones can range from small, pea-sized pellets to large chunks of ice capable of damaging roofs, breaking windows, destroying crops, and injuring people and animals. Their size depends on several factors, including the strength of the storm’s updrafts, the availability of liquid water, the temperature inside the cloud, and how long the stones remain suspended in the storm.
Understanding how hail forms reveals the remarkable processes taking place inside thunderstorms, where powerful air currents, supercooled water, and freezing temperatures combine to produce one of nature’s most destructive forms of precipitation.
What is hail?
Hail is solid precipitation consisting of balls or irregular lumps of ice called hailstones. It forms in thunderstorms, particularly those with strong vertical air currents that can carry ice particles upward through regions of abundant liquid water.
Although hail and other forms of frozen precipitation all involve ice, they develop under different atmospheric conditions. Snow forms when ice crystals grow in clouds and join together, often producing the familiar flakes that fall during winter weather. Sleet forms when raindrops freeze into small ice pellets as they fall through a sufficiently deep layer of cold air near the ground. Freezing rain, by contrast, remains liquid as it falls but freezes when it touches a surface that is at or below freezing.
Hail develops primarily inside thunderstorms, where strong updrafts can keep growing ice particles aloft. These particles collect water and grow much larger than ordinary ice crystals. Once a hailstone becomes too heavy for the surrounding air currents to support, gravity pulls it toward the ground.
Hail does not require freezing temperatures at the surface. It can fall during warm weather, even when the air near the ground feels hot. The critical conditions for hail formation exist higher in the thunderstorm, where temperatures can be well below freezing.
How hailstones begin to form
Hail formation starts inside a thunderstorm cloud, where rising air carries moisture from lower levels of the atmosphere into colder regions.
Thunderstorms develop when warm, moist air rises into the atmosphere. As the air rises, the surrounding atmospheric pressure decreases, allowing the air to expand and cool. When it cools sufficiently, water vapor condenses into tiny liquid droplets, forming a cloud. If the rising air continues upward, the cloud can grow into a tall cumulonimbus cloud, the type associated with thunderstorms.
Within these clouds, temperatures vary with altitude. The lower portions may contain liquid water, while the upper portions contain ice crystals, snow particles, and other frozen precipitation. Between these regions, liquid water can persist even when the surrounding air is below freezing.
This extremely cold liquid water is called supercooled water. It remains liquid below 32°F (0°C) because freezing does not always begin immediately when water reaches that temperature. The formation of ice often requires a suitable particle or surface on which ice crystals can start to develop.
Supercooled droplets are essential to the growth of many hailstones. When a droplet collides with an existing ice particle and freezes, it adds more ice to that particle. As collisions continue, the particle grows into a larger hailstone.
The first ice particle may originate as a frozen cloud droplet, a small ice crystal, or another piece of frozen precipitation. Once it enters a region containing abundant supercooled water, it can begin growing rapidly.
The role of updrafts inside thunderstorms
The strength of a thunderstorm’s updraft is one of the most important factors determining whether hailstones can grow large.
An updraft is a rising current of air within a cloud. It develops when air that is warmer and less dense than its surroundings rises through the atmosphere. In a thunderstorm, updrafts can carry moisture, cloud droplets, and ice particles thousands of feet upward.
As a developing hailstone moves through the cloud, the updraft can keep it suspended against the downward pull of gravity. This allows the stone to remain inside the storm long enough to collect additional water and grow.
The stronger the updraft, the greater the potential for a hailstone to remain aloft while it grows. However, updraft strength alone does not determine hail size. The storm must also supply enough supercooled liquid water, and the hailstone must follow a path that allows it to collect that water efficiently.
A weak or short-lived updraft may allow only small ice particles to develop before they fall. A powerful updraft can support larger stones for longer periods, provided the other conditions for growth are favorable.
In some thunderstorms, the rising air is organized into a rotating column known as a mesocyclone. This feature is associated with supercell thunderstorms, a type of storm that can produce particularly large hail. Rotation can help organize the storm’s airflow and maintain a persistent updraft, creating conditions that favor hail growth.
However, not every hail-producing thunderstorm is a supercell. Other types of thunderstorms can also generate hail when their updrafts and internal water and temperature conditions are suitable.
How hailstones grow inside a cloud
Once an ice particle begins collecting supercooled water, its growth depends on how droplets collide with it and how the water freezes.
When a hailstone encounters a supercooled droplet, the droplet may freeze on contact. Repeated collisions add successive amounts of ice to the growing stone. Over time, the particle becomes larger and heavier.
The freezing process also affects the hailstone’s internal structure. If the water freezes relatively slowly, some of the heat released during freezing can escape into the surrounding air. The ice may form with a relatively clear appearance because fewer air bubbles become trapped inside it.
If the water freezes rapidly, some air can become trapped within the newly formed ice, producing a cloudy or opaque layer. Differences in freezing conditions can create variations in the density and appearance of a hailstone’s ice.
Hailstones do not always grow as neat, round spheres. Their shapes depend on how they move through the storm, how water strikes their surfaces, and how ice accumulates. Some are roughly spherical, while others develop irregular lobes, flattened areas, or jagged projections.
The amount of water available for freezing is particularly important. A storm with strong updrafts but relatively little supercooled liquid water may not produce exceptionally large hail. Conversely, a storm that combines powerful updrafts with abundant supercooled water can support substantial growth.
Hailstones may also collide with one another or with other ice particles. These interactions can alter their shapes, cause them to break apart, or change the paths they follow through the cloud.
Why some hailstones have layers of ice
Many hailstones contain alternating layers of clear and cloudy ice. When a hailstone is cut open, these layers can sometimes resemble the rings of an onion.
These layers develop because the stone experiences changing conditions as it grows. Different parts of a thunderstorm have different temperatures, concentrations of liquid water, and airflow patterns. A hailstone moving between these regions may encounter conditions that cause water to freeze at different rates.
When supercooled droplets freeze relatively slowly, air can escape more readily, producing clearer ice. When they freeze quickly, air bubbles may become trapped, creating a more opaque layer.
A hailstone can therefore accumulate alternating bands of relatively clear and cloudy ice as it travels through different parts of the storm.
However, not all hailstones have distinct layers. Some grow primarily under one set of conditions, producing a more uniform internal structure. Others have complicated patterns that do not form a simple sequence of alternating rings.
The visible layers provide clues about the conditions under which a hailstone grew, but they do not necessarily represent a precise count of the number of times it moved up and down through a thunderstorm. Each layer reflects the local conditions and growth processes that affected the ice.
Does a hailstone travel up and down inside a thunderstorm?
Hailstones can move through different altitudes within a thunderstorm, but the idea that every hailstone repeatedly rises and falls in a simple cycle is misleading.
Their movement depends on the storm’s three-dimensional airflow, the strength of its updrafts, the hailstones’ sizes, and their positions within the cloud. Some stones may follow paths that take them through several regions of the storm, while others may grow along more direct trajectories.
A developing hailstone can remain within a strong updraft as it collects supercooled water. As it becomes larger, its downward speed relative to the surrounding air generally increases. Eventually, the stone may move downward through the storm even while air continues to rise around it.
Some hailstones grow while moving between regions with different temperatures and water concentrations. Others may be carried along curved or sloping paths as the storm’s airflow changes.
The important factor is not the number of times a hailstone moves upward or downward. It is the combination of its trajectory, the availability of liquid water, and the time it spends in conditions that support growth.
Why hailstones eventually fall to the ground
A hailstone falls when the downward force of gravity exceeds the upward aerodynamic support provided by the surrounding airflow.
As a hailstone grows, its weight increases. Larger stones generally fall faster through the air than smaller ones because gravity pulls them downward more strongly relative to the drag acting on them.
An updraft may support a small hailstone but fail to keep a larger one suspended. Once the stone’s downward motion carries it out of the strongest rising air, it can begin falling through the cloud and toward the ground.
The process is not always a sudden transition. A hailstone may gradually descend as its size increases or as the updraft weakens. It may also move out of the updraft because of changes in the storm’s airflow.
During its descent, the stone can continue colliding with supercooled droplets if it remains in a region where liquid water is present. However, growth eventually slows or stops as it leaves those conditions.
What happens next depends on the temperature profile between the cloud and the ground. If the hailstone passes through a sufficiently warm layer of air, some of its ice may melt. Small hailstones are especially susceptible to melting before they reach the surface.
Large hailstones have more ice to lose during their descent and may retain a solid core even after passing through warm air. This is why hail can reach the ground on a warm day without requiring freezing temperatures at ground level.
What determines the size of a hailstone?
Hailstone size depends on several interacting factors, and no single feature of a thunderstorm determines the outcome.
Updraft strength influences how long a growing hailstone can remain suspended. Stronger updrafts can support larger stones, although the relationship also depends on the stone’s density, shape, and surrounding airflow.
The supply of supercooled water determines how much material is available for ice growth. A hailstone cannot become exceptionally large without collecting substantial amounts of liquid water that can freeze onto its surface.
Time spent in favorable conditions matters because growth requires repeated collisions between the hailstone and supercooled droplets. A stone that spends longer in a region with abundant liquid water may accumulate more ice.
The storm’s temperature structure affects where supercooled water exists and how quickly droplets freeze. These conditions influence both the rate of growth and the internal structure of the hailstone.
The hailstone’s path through the storm determines which regions it encounters. Two ice particles in the same thunderstorm may experience different growth histories and emerge at very different sizes.
Large hail is therefore the result of a combination of favorable conditions rather than simply a storm being unusually tall or cold.
Hailstones range from small pellets to stones several inches across. Exceptionally large hail can occur when a storm maintains strong, organized updrafts and supplies abundant supercooled water over a sufficiently long period.
Why hail can fall during warm weather
Hail can occur during spring, summer, or other relatively warm periods because its formation takes place high inside a thunderstorm rather than at the ground.
In a typical thunderstorm, temperatures decrease with altitude through much of the troposphere, the lowest major layer of Earth’s atmosphere. Air near the ground can be warm and humid while the air several miles above it remains far below freezing.
Strong updrafts carry moisture into these colder regions, where ice particles develop and grow. The resulting hailstones then fall through the warmer air below.
Whether they reach the ground intact depends partly on their size and the depth and temperature of the warm layer. Small hailstones may melt completely before reaching the surface. Larger stones may melt only partially, allowing ice to survive all the way down.
This also explains why hail does not necessarily indicate that the air near the ground is cold. The atmospheric conditions that produce hail can exist above a warm surface layer.
How hail differs from graupel and sleet
Hail, graupel, and sleet are all forms of frozen precipitation, but they form through different processes.
Hail develops when ice particles grow inside thunderstorms by collecting supercooled liquid water. The resulting stones can become large enough to cause substantial damage.
Graupel forms when supercooled droplets freeze onto snowflakes or other ice crystals, coating them in a layer of rime. The resulting particles are generally small, soft, and often crumbly. Graupel can form in a wider range of cold-cloud conditions than those required for large hailstones.
Sleet forms when snow or partially melted snow passes through a layer of air warm enough to melt it and then enters a sufficiently deep layer of subfreezing air near the ground. The liquid precipitation refreezes into small ice pellets before reaching the surface.
The distinction matters because each type of precipitation reveals something different about the atmosphere. Hail indicates that ice has grown within a storm under conditions that support the collection of supercooled water, while sleet indicates a particular arrangement of above-freezing and below-freezing air layers.
Why hail is dangerous
Hail can damage property, destroy crops, harm animals, and injure people. The severity of the damage depends largely on the size, density, speed, and quantity of the falling stones, as well as the vulnerability of whatever they strike.
As hailstones grow, their mass increases substantially. Their falling speed also generally increases with size, although air resistance and shape affect their motion. Larger stones can therefore deliver much more energy on impact than small ones.
Hail can dent vehicles, crack windshields, damage roof shingles, break skylights, and injure people caught outdoors. Repeated impacts can strip leaves from plants, damage fruit, flatten crops, and harm livestock.
Even relatively small hail can become hazardous when it falls heavily enough to cover roads and sidewalks. Accumulated hail can make surfaces slippery, obstruct drainage, and reduce visibility for drivers.
Large hail is especially dangerous because it can cause serious injuries. During a hailstorm, people should seek shelter inside a sturdy building or an enclosed vehicle when available. Staying away from windows and exposed areas helps reduce the risk of injury from impacts and broken glass.
Hail damage also depends on the storm’s movement and duration. A brief burst may produce little accumulation, while a prolonged or repeated hailfall can cause extensive damage across an area.
Where hail is most common
Hail can occur wherever thunderstorms develop under suitable atmospheric conditions, but some regions experience it more frequently than others.
In the United States, hail is a familiar hazard across parts of the Great Plains, the Midwest, and other areas where thunderstorms can develop strong updrafts. The central Plains are particularly well known for severe thunderstorms capable of producing large hail.
The region east of the Rocky Mountains often experiences combinations of atmospheric instability, moisture, and wind patterns that favor strong thunderstorm development. Atmospheric instability refers to a condition in which rising air can remain warmer and less dense than its surroundings, allowing it to continue rising and potentially strengthen an updraft.
Hail also occurs outside the United States. Mountainous regions and areas with frequent thunderstorms can experience hail when the necessary cloud and airflow conditions are present.
Geography alone does not determine whether hail will form. Local weather conditions, the structure of individual thunderstorms, and the availability of supercooled water all influence where hail develops and how large it becomes.
How meteorologists predict hail
Meteorologists assess several atmospheric conditions to estimate whether thunderstorms are likely to produce hail and whether that hail could become large.
Weather observations and forecast models help reveal the atmosphere’s temperature profile, moisture content, instability, and wind patterns. These factors indicate whether strong updrafts may develop and whether the storm could contain regions with abundant supercooled liquid water.
Weather radar provides additional information about precipitation inside thunderstorms. Radar can reveal the location, intensity, and structure of storms, while specialized radar measurements can help meteorologists assess the distribution of precipitation and identify features associated with potentially severe hail.
However, radar does not directly measure the size of every hailstone inside a cloud. Meteorologists combine radar observations with atmospheric measurements, storm structure, and other evidence to estimate hail potential.
Forecasting hail size remains challenging because relatively small differences in a storm’s updraft strength, airflow, and liquid-water distribution can influence how large its hailstones become. A thunderstorm may produce different hail sizes in different locations, and conditions can change as the storm evolves.
Hail forecasts therefore describe the potential for hail and the likelihood of damaging impacts rather than guaranteeing the exact size of the stones that will reach the ground.
Why hail is an important part of thunderstorm science
Hail provides a window into the complex processes occurring inside thunderstorms. Its formation demonstrates how atmospheric moisture, temperature, gravity, and powerful air currents interact to shape precipitation.
A hailstone’s size and internal structure reflect its growth history. The ice records changing conditions as water freezes onto its surface, while the stone’s final size reflects the balance between its growth, the forces supporting it in the cloud, and the time it spends in favorable conditions.
Hail also illustrates why weather at the ground does not always reveal what is happening higher in the atmosphere. A warm afternoon can coincide with temperatures well below freezing inside a towering thunderstorm, allowing ice to grow and fall through the warm air below.
Ultimately, hail forms when a thunderstorm creates the right combination of supercooled liquid water, growing ice particles, and strong air currents. The hailstone grows as water freezes onto it, falls when the surrounding airflow can no longer support it, and may reach the ground intact if it does not melt completely along the way.
