Hailstorms form when powerful thunderstorms carry raindrops and tiny ice particles into regions of the atmosphere where temperatures are below freezing. Inside these clouds, water freezes onto ice particles, allowing them to grow into solid balls or irregular lumps of ice. When the stones become too heavy for the storm’s rising air currents to support, they fall to the ground as hail.
Although hail often accompanies heavy rain, lightning, and strong winds, it forms through a distinct process that depends on the storm’s internal structure. The size of the hailstones depends on factors such as the amount of supercooled water in the cloud, the strength of its updrafts, and how long the growing stones remain suspended in the storm.
Understanding hailstorms requires looking inside thunderclouds, where liquid water and ice can coexist at temperatures well below freezing.
What conditions cause a hailstorm?
Hail develops most often in strong thunderstorms called cumulonimbus clouds. These towering clouds form when warm, moist air rises into cooler layers of the atmosphere. As the air rises, it expands because atmospheric pressure decreases with altitude. Expansion cools the air, causing water vapor to condense into tiny liquid droplets and form a cloud.
Several ingredients help thunderstorms grow strong enough to produce hail. Warm, humid air near the ground supplies moisture and energy. Cooler air higher in the atmosphere creates a temperature difference that can make the atmosphere unstable, allowing rising air to accelerate. A mechanism that lifts the air, such as a weather front, a mountain slope, or the convergence of surface winds, can help initiate the storm.
Once a thunderstorm develops, strong upward-moving currents called updrafts can carry water droplets far above the freezing level. This is the altitude where the surrounding air temperature reaches 0 degrees Celsius, or 32 degrees Fahrenheit. The freezing level varies with weather conditions and location, so it does not occur at the same height in every storm.
A high freezing level does not automatically prevent hail, nor does a low one guarantee it. What matters is whether the storm can transport water into sufficiently cold air and provide the conditions needed for ice to grow before gravity pulls it downward.
Not every thunderstorm produces hail. Many storms have updrafts that are too weak, contain too little suitable liquid water, or do not maintain favorable conditions long enough for hailstones to grow. Hail-producing storms require a particular combination of atmospheric instability, moisture, and storm dynamics.
How water stays liquid below freezing
One of the most important ingredients in hail formation is supercooled water.
Under ordinary conditions, pure water freezes at 0°C (32°F). However, tiny water droplets in clouds can remain liquid at temperatures below this point. These droplets are known as supercooled water droplets.
Freezing often begins when a droplet encounters a suitable particle or an existing ice crystal that provides a surface for ice to form. In a cloud containing many small droplets, some may remain liquid well below freezing while others freeze or join existing ice particles.
Supercooled droplets are especially important because they provide the raw material for hailstone growth. When a growing ice particle collides with these droplets, the droplets can freeze onto its surface. This process is called accretion.
As more droplets freeze onto the particle, it becomes larger and heavier. Repeated collisions can transform a tiny ice particle into a hailstone large enough to reach the ground.
The temperature of the surrounding air influences how the collected water freezes. In colder parts of a thunderstorm, water may freeze quickly. In somewhat warmer, subfreezing regions, freezing may occur more slowly, sometimes allowing a thin layer of liquid water to spread across the stone before solidifying. These different freezing conditions help produce variations in hailstone structure.
How hailstones grow inside thunderclouds
Hailstone growth begins when small ice particles, frozen droplets, or other ice particles encounter supercooled water within a thunderstorm. Once an ice particle starts collecting liquid droplets, it can grow through repeated collisions as air currents move it through the cloud.
The process depends on a continuous supply of suitable water. A storm may contain abundant moisture, but a growing hailstone must pass through regions where liquid droplets are available and collide with them frequently enough to gain mass.
As the stone grows, its path through the cloud becomes increasingly important. Rising air can carry it upward, while gravity pulls it downward. Turbulent air currents may move it sideways or alter its trajectory. Some stones pass through several regions of the storm, encountering different temperatures and concentrations of liquid water along the way.
Hailstones do not all follow the same path. Some grow during a relatively short journey through a favorable region of the cloud. Others may remain in strong updrafts long enough to collect more water and become much larger.
The stone’s surface conditions also influence how it grows. When collected droplets freeze rapidly, they can trap small air bubbles within the ice, producing a cloudy or opaque layer. When water freezes more gradually, air bubbles may escape more easily, creating clearer ice.
These differences can create alternating layers of cloudy and clear ice in some hailstones. However, the familiar image of a hailstone growing through a perfectly regular series of rings is an oversimplification. Real hailstones are often irregular, and their internal structures reflect changing conditions and complex movements within the cloud.
Why strong updrafts produce larger hailstones
An updraft is a current of air moving upward through a thunderstorm. Its strength is one of the main factors controlling how large hailstones can become.
As hailstones gain mass, gravity pulls them downward more strongly relative to the upward support provided by the air. A sufficiently strong updraft can keep a growing stone suspended, allowing it to collect additional supercooled droplets.
If the updraft weakens or the stone becomes too large for the upward airflow to support, the stone begins to descend relative to the cloud. It may continue growing while passing through other regions containing supercooled water, but it will eventually fall out of the storm.
Large hail is therefore associated with thunderstorms capable of maintaining strong upward currents and supplying abundant liquid water in cold regions of the cloud. These storms can sustain hailstone growth for longer periods than weaker storms.
However, updraft strength alone does not determine hail size. A powerful updraft cannot produce large hail without sufficient supercooled water, and a stone must encounter that water efficiently. The storm’s temperature profile, turbulence, internal airflow, and the time available for growth all influence the final result.
Some of the most favorable environments for large hail occur in organized thunderstorms known as supercells. These storms contain a persistent, rotating updraft that can maintain a relatively stable structure for an extended period. Their organization can support sustained hail growth, although not every supercell produces large hail and hail can also occur in other types of thunderstorms.
Why hailstones have different sizes and shapes
Hailstones range from small pellets to large, damaging chunks of ice. Their final size reflects the conditions they experience while growing and falling.
Small hailstones may form when ice particles collect limited amounts of supercooled water or spend little time in favorable growth regions. Larger hailstones generally require more extensive growth, which means sustained access to liquid water and airflow capable of keeping them aloft.
Hailstones can also have irregular shapes. Their surfaces may develop unevenly because they collide with droplets at different angles, rotate through turbulent air, or grow at different rates across their surfaces. Collisions between ice particles can further alter their shapes.
Some stones develop rough, knobby surfaces, while others are comparatively smooth or rounded. Internal air pockets, cracks, and layers of different ice textures may form as the stone grows.
The largest stones are not necessarily produced by simply allowing a small stone to travel upward and downward repeatedly. Hailstones can move along complicated paths through a storm, and researchers cannot always reconstruct an individual stone’s complete history from its final appearance.
Even so, the central principle remains clear: larger hailstones generally require more time and more opportunities to collect freezing water, supported by a storm with favorable airflow and moisture conditions.
Why hail falls to the ground
Hailstones eventually fall when gravity overcomes the upward support of the surrounding air. As a stone descends, it encounters air resistance, which slows its fall. Its falling speed depends on its size, shape, density, and the properties of the surrounding air.
A stone may leave the cloud while still growing, particularly if it passes through a region containing supercooled droplets. But its opportunity to collect additional water usually diminishes as it moves into warmer, lower layers of the atmosphere.
What happens next depends on the temperature of the air between the cloud and the ground. If the hailstone falls through a sufficiently deep layer of warm air, its surface melts. Small stones can melt completely before reaching the ground, while larger stones may retain an icy core because they have more ice relative to their surface area.
This is one reason hail can reach the ground even when the air near the surface is well above freezing. A large stone may take long enough to melt that some of it survives the journey.
The height of the freezing level matters, but it is only part of the picture. A long, warm descent favors melting, while a shorter or colder descent can allow more hail to survive. The stone’s initial size and the temperature profile along its path also play important roles.
Hail can occur during warm-weather thunderstorms because the upper portions of these clouds extend into air that is well below freezing. The temperature at ground level does not reveal the conditions where the hail formed.
How hailstorms differ from sleet and freezing rain
Hail, sleet, and freezing rain all involve frozen precipitation, but they form through different processes.
Hail grows inside thunderstorms when ice particles collect supercooled liquid water. Strong updrafts and suitable cloud conditions allow these particles to become relatively large before they fall.
Sleet, also called ice pellets, usually forms when snowflakes descend through a layer of air warm enough to melt them partially or completely, then enter a sufficiently deep layer of subfreezing air near the ground. The resulting drops refreeze into small ice pellets before reaching the surface.
Freezing rain forms when snow or ice melts into liquid drops in a warm layer aloft and then falls through a shallow layer of subfreezing air. The drops remain liquid until they strike a surface that is at or below freezing, where they freeze and create a coating of ice.
The distinctions are important because the hazards differ. Hail can damage roofs, vehicles, crops, and exposed equipment through direct impacts. Sleet can make roads slippery, while freezing rain can create dangerous layers of ice on roads, trees, power lines, and other surfaces.
Unlike sleet and freezing rain, hail is strongly associated with the vigorous vertical air motion of thunderstorms.
Why hailstorms can cause serious damage
The danger from hail comes largely from the impact of falling ice. A hailstone carries kinetic energy, which depends on its mass and speed. Larger stones generally carry much more energy than smaller ones, making them more capable of breaking glass, denting metal, damaging roofs, and injuring people or animals.
Hail can also destroy crops by tearing leaves, bruising fruit, breaking stems, and damaging developing plants. Even when a plant survives, damage to its leaves and stems can interfere with growth and reduce its ability to produce food through photosynthesis.
Vehicles and buildings are especially vulnerable when hailstones are large or when a storm produces hail over a broad area. Skylights, windows, siding, and roofing materials may be damaged, and repeated impacts can worsen existing weaknesses.
Hailstorms may also occur alongside damaging winds, lightning, and heavy rainfall. These hazards can compound one another, creating dangerous conditions for people outdoors or traveling on roads.
Not every hailstorm causes major damage. Many produce small stones that melt quickly or reach the ground without significant consequences. The severity of a hail event depends on the size and quantity of the stones, the area affected, the vulnerability of exposed property and crops, and the other weather conditions accompanying the storm.
Where hailstorms are most common
Hail occurs in many parts of the world, but it is particularly associated with regions where thunderstorms frequently develop in atmospheres that support strong updrafts.
In the United States, the central Great Plains and surrounding areas are well known for severe hailstorms. Warm, moist air from the Gulf of Mexico can interact with cooler, drier air from other regions, helping create the instability and storm dynamics needed for powerful thunderstorms. These conditions can support storms capable of producing large hail.
Hail is not limited to the Great Plains. Thunderstorms can produce hail across much of the country, including parts of the Midwest, the Southeast, the Mountain West, and other regions when the atmospheric conditions are favorable.
Elevation, season, and local weather patterns affect where hail is most likely. Higher terrain can influence the development of rising air, while seasonal changes alter the availability of moisture, atmospheric instability, and cold air aloft.
A region’s typical ground-level temperature does not determine whether hail can occur. The key is whether thunderstorms develop a sufficiently cold, moist, and dynamically favorable environment for ice to grow and survive its descent.
Can scientists predict hailstorms?
Meteorologists can identify atmospheric conditions that favor hail, but predicting the exact location, timing, and size of hailstones remains challenging.
Weather observations and numerical forecast models help forecasters assess atmospheric instability, moisture, wind patterns, freezing levels, and the likelihood of strong thunderstorms. Radar can reveal storm structure, precipitation patterns, and other features that help meteorologists evaluate the potential for hail.
However, a radar image does not always reveal the exact size of hailstones reaching the ground. Radar measurements are influenced by factors such as the amount of precipitation, the size and composition of particles, and the distance between the radar and the storm. Hail reports and observations can therefore provide important additional information.
Forecasting the potential for hail is different from predicting the precise size of individual stones. A storm may have conditions favorable for large hail without producing it, while small changes in the storm’s internal airflow and water supply can influence how much the hailstones grow.
Warnings become especially important when a thunderstorm is expected to produce damaging hail. People can reduce their exposure by moving indoors, staying away from windows, and protecting themselves rather than attempting to observe the storm from outside.
What hailstones reveal about thunderstorms
Hailstones are products of the complex interactions between water, ice, gravity, and moving air. Their growth provides a useful illustration of how much can happen inside a thunderstorm that appears, from the ground, to be a single dark cloud.
Each stone’s size and structure reflect the conditions it experienced during its journey through the storm. Changes in temperature affect how water freezes. The availability of supercooled droplets determines how much material can accumulate. Updrafts and turbulence influence where the stone travels and how long it can continue growing.
Scientists use observations of hail, radar measurements, and studies of thunderstorm dynamics to improve their understanding of these processes. Although the precise growth history of an individual stone can be difficult to determine, the physical mechanisms responsible for hail formation are well established.
Ultimately, hailstorms occur when powerful thunderstorms provide the right combination of cold air, supercooled liquid water, and upward-moving currents. Tiny ice particles collect freezing droplets, grow into heavier stones, and eventually fall when the storm can no longer support them. The same process that creates small pellets of ice can, under especially favorable conditions, produce large hail capable of causing substantial damage.