Hail forms when strong updrafts inside thunderstorms carry water droplets high enough to freeze, then keep those frozen particles suspended long enough for them to grow by collecting more supercooled water. The process requires a specific combination of atmospheric instability, moisture, and vertical wind patterns, which is why only a fraction of thunderstorms ever produce hailstones. What reaches the ground can range from pea-sized pellets to softball-sized chunks of ice, depending on how long the stone stays aloft and how it accumulates mass on the way.
How a Thunderstorm Builds a Hailstone
Every hailstone starts as a small frozen particle, sometimes a graupel pellet, a frozen raindrop, or even a clump of ice crystals, that gets caught in a thunderstorm’s updraft. Updrafts in severe thunderstorms can exceed speeds of 100 miles per hour, easily strong enough to keep a growing chunk of ice suspended thousands of feet above the ground. The upper reaches of a tall cumulonimbus cloud are extremely cold, often below minus 30 or 40 degrees Celsius, but the cloud also contains enormous quantities of supercooled liquid water: droplets that remain liquid despite being well below the freezing point. When the small ice particle passes through regions rich in these supercooled droplets, the droplets freeze onto its surface and the stone grows.
The critical factor is time. A weak updraft drops the particle quickly, and it either melts on the way down or arrives as a small, harmless pellet. A powerful, sustained updraft recirculates the stone repeatedly through the cloud’s growth zone, or holds it steady in a region with abundant supercooled water. Each pass or each extra second suspended adds another coat of ice. Once the stone becomes heavy enough that the updraft can no longer support it, gravity wins and the hailstone falls.
Supercell thunderstorms are the most prolific hail producers. These storms feature a rotating updraft called a mesocyclone, which keeps the core of rising air organized and persistent. One study analyzing a large-hail supercell over the Adriatic found that the entire storm system exhibited counterclockwise rotation consistent with mesocyclone characteristics, with the updraft structure remaining coherent long enough to support significant hail growth.1Atmospheric Research. Analysis of the development mechanisms of a large-hail storm event on the Adriatic Sea That rotational structure separates the updraft from the downdraft, preventing the storm from choking itself off with its own rain and hail. A non-rotating thunderstorm tends to collapse more quickly, giving hailstones less time to grow.
Dry Growth and Wet Growth
Not all ice accumulation on a hailstone looks or behaves the same way. Meteorologists distinguish between two modes of growth, and the difference shows up clearly when you slice a large hailstone in half.
In dry growth, supercooled droplets freeze almost instantly on contact with the hailstone’s surface. The stone’s surface temperature stays well below freezing, so each droplet solidifies before the next one arrives. Because the freezing is rapid and somewhat chaotic, tiny air bubbles get trapped in the ice, giving the resulting layer a milky, opaque appearance.2Atmospheric Research. Shape of hail and its thermodynamic characteristics related to records in Catalonia If you have ever seen a hailstone that looked white or frosty, you were looking at ice that formed under dry growth conditions.
Wet growth is different. Here, the hailstone collects supercooled water faster than it can dissipate the heat released by freezing. The surface temperature rises to around 0°C, and a thin film of liquid water persists on the outside of the stone. Incoming droplets blend into this liquid layer rather than freezing on contact. The water eventually freezes more slowly from the inside out, producing clear ice with very few air bubbles.2Atmospheric Research. Shape of hail and its thermodynamic characteristics related to records in Catalonia Think of the glassy, transparent ice you see in a freezer-made ice cube versus the cloudy ice from a fast-freeze tray: the hailstone equivalent is the same distinction, driven by freezing speed.
A hailstone does not stay in one growth mode for its entire life. As it moves through different parts of the cloud, encountering varying temperatures and liquid water concentrations, it can switch back and forth between dry and wet growth many times.
Why Hailstones Have Rings Like a Tree
Slice a large hailstone through the middle and you often see concentric rings of alternating cloudy and clear ice, looking something like a cross-section of an onion. For decades, the popular explanation was that each ring represented a separate trip through the cloud: the stone was lofted up, fell partway down, got caught in the updraft again, and so on. Each “ride” through the growth zone supposedly left a new ring.
That explanation turns out to be an oversimplification. Research on hailstone density and structure has shown that alternating layers of opaque and clear ice can form from transitions between dry and wet growth without requiring repeated up-and-down cycling. Even minor fluctuations in the cloud’s liquid water content can push a growing hailstone from one growth mode to the other, producing a new layer each time.3Quarterly Journal of the Royal Meteorological Society. The density and structure of hailstones The surface temperature during wet growth hovers near 0°C, so it does not take a dramatic environmental shift to tip the balance. A hailstone can acquire many alternating layers while remaining at roughly the same altitude in a sustained updraft, simply because the supply of supercooled water around it fluctuates.
This matters because the old “elevator ride” model implied that hailstones needed many round trips to get big, which would require implausibly long times aloft for the largest stones. The growth-transition model is more physically plausible: a stone held in a strong, steady updraft passes through slightly different pockets of cloud water, switching growth modes as conditions shift around it.
The Atmospheric Conditions Hail Needs
Not every thunderstorm makes hail, and the storms that do are not all the same type. A global analysis of severe hailstorm environments identified five distinct categories, each with a different combination of instability, moisture, and wind shear.4Geophysical Research Letters. How Many Types of Severe Hailstorm Environments Are There Globally?
Two of those categories occur over tropical plains and hills. These environments feature high convective instability and plenty of moisture but relatively low vertical wind shear, meaning the wind speed and direction do not change dramatically with altitude. Storms in these settings can be explosive and tall, lofting water to great heights, but they tend to be shorter-lived because the lack of shear means the updraft and downdraft interfere with each other.
A third type is the classic supercell environment, found most often over mid-latitude plains like the central United States or parts of Argentina. These settings combine moderate instability and moisture with strong vertical wind shear, which tilts the storm and separates the updraft from the downdraft. This is the recipe for long-lived rotating storms capable of producing the largest hailstones.
The remaining two categories appear at higher latitudes or over elevated terrain. These environments have moderate wind shear but lower instability, less moisture, and a lower melting level, meaning the freezing line sits closer to the ground. A lower melting level means hailstones have less distance to travel through warm air before hitting the surface, so even moderately sized stones can arrive without melting away entirely.
This last point is often underappreciated. Two storms producing identical hailstones at cloud level can deliver very different hail at the ground, entirely because of the depth of warm air below the cloud base.
What Happens on the Way Down
Once a hailstone leaves the updraft, it begins falling through progressively warmer air. Melting starts at the surface and works inward, and the rate depends on the stone’s size, the temperature of the air it passes through, and the relative humidity. A large stone melts more slowly relative to its mass because it has less surface area per unit of volume. A small stone, by contrast, may melt completely before reaching the ground, arriving as a large raindrop rather than ice.
This melting process reshapes the size distribution of hail that actually reaches the surface. Modeling of how exponentially distributed hailstone sizes change during descent shows that smaller stones are disproportionately removed by melting, shifting the ground-level distribution toward larger stones and making the remaining population look different from what existed inside the cloud.5Atmospheric Research. The influence of melting on hailstone size distribution In practical terms, when you collect hail after a storm, you are seeing a filtered version of what the storm actually produced. The smallest stones never made it.
Humidity matters too. In dry air, evaporative cooling at the hailstone’s surface slows the melting process, effectively giving stones a longer survival window. In humid air near the surface, that cooling effect weakens, and melting accelerates. This is one reason why the same storm can drop large hail in one location and only rain a few miles away: local variations in temperature and humidity along the hailstone’s fall path make the difference.
How Climate Change Is Shifting Hail Patterns
Climate change complicates hail forecasting because it pulls the atmosphere in competing directions. A warmer atmosphere holds more moisture and more energy, which should fuel stronger updrafts and bigger hailstones. But a warmer atmosphere also raises the melting level, meaning hailstones have to fall through a deeper layer of above-freezing air before reaching the ground, which melts them down or eliminates them altogether. Which effect wins depends on the type of storm.
A study modeling hailstorms under anthropogenic climate change found strikingly different results depending on the storm’s weather system. Hailstorms developing in frontal systems showed an increase of over 110% in large hail occurrences (stones bigger than 2.5 centimeters), driven by stronger convective intensity and wider updrafts. Storms developing in Great Plains low-level jet systems, by contrast, showed less than a 30% increase in large hail.6Earth’s Future. Contrasting Responses of Hailstorms to Anthropogenic Climate Change in Different Synoptic Weather Systems The frontal storms saw a smaller increase in warm cloud depth, meaning melting had less of a compensating effect. The low-level jet storms gained more warm cloud depth, partially canceling out the benefit of stronger updrafts.
The takeaway is that blanket statements about climate change producing “more hail” or “less hail” miss the point. The answer varies by region, by season, and by the type of weather pattern producing the storm. Some areas may see more frequent giant hail while total hail days actually decrease, because the conditions that support moderate hail erode while the conditions for extreme hail intensify.
Can We Stop Hail With Cloud Seeding
People have tried to suppress hail for over half a century, most commonly by seeding thunderstorms with silver iodide. The idea is straightforward: introduce extra ice nuclei into the cloud so that the available supercooled water gets distributed across more ice particles, each of which grows smaller than it would have otherwise. Instead of a few large hailstones, you get many small ones that are more likely to melt before reaching the ground, or at least do less damage.
The results have been mixed. A review of hail suppression projects conducted over nearly three decades concluded that silver iodide seeding suppresses hail in some storms but not in others, and the effect is largely independent of the amount of silver iodide used, provided a reasonable threshold is exceeded.7The Journal of Weather Modification. How Silver Iodide seeding suppresses hail In other words, dumping more silver iodide into a storm does not necessarily produce a bigger reduction in hail. Whether seeding works seems to depend more on the storm’s characteristics than on the dose.
More encouraging results have come from specific regional programs. A study of ground-based silver iodide generators in France and Spain found that when seeding started three hours before hail reached the ground, using a network of generators spaced about 10 kilometers apart, the hailfall energy on the most severe hail days dropped by roughly half.8Atmospheric Research. Hail prevention by ground-based silver iodide generators: Results of historical and modern field projects That is a substantial reduction, but it required a dense, well-organized network and enough lead time to seed the developing storm areas. Those conditions are not always achievable, and the results have not been consistently replicated across all hail climates.
The scientific community remains divided on whether cloud seeding for hail suppression is reliable enough to justify widespread adoption. Many agricultural regions continue to fund seeding programs on the logic that even partial reduction in crop damage pays for itself, while atmospheric scientists argue the evidence base is still too thin for confident conclusions about effectiveness.
What Hail Does When It Hits
The damage potential of a hailstone rises dramatically with size, far more than you might expect from a simple increase in diameter. A 45-millimeter hailstone (roughly the size of a golf ball) hitting a standing seam metal roof at terminal velocity delivers about 290% more impact energy than a 32-millimeter stone, even though the diameter only increased by about 40%.9Structures. Dynamic response of standing seam metal roof systems under hail disaster: Experimental and numerical simulation This is because kinetic energy depends on both mass and velocity, and both increase with diameter: a bigger stone is heavier and falls faster.
Roofing materials are tested against hail using a classification system that simulates terminal-velocity impacts. The ratings work as follows:
- Class 1: Withstands a single impact from a 1.25-inch ice sphere.
- Class 2: Withstands a single impact from a 1.5-inch ice sphere.
- Class 3: Withstands a single impact from a 1.75-inch ice sphere.
- Class 4: Withstands a single impact from a 2.0-inch ice sphere.
These ratings are based on whether the roofing material cracks or breaks under the simulated impact.10International Journal of Disaster Risk Reduction. Hail hazard modeling with uncertainty analysis and roof damage estimation of residential buildings in North America A Class 4 roof can handle a 2-inch hailstone, which is serious hail, roughly the size of a hen’s egg. But the most extreme storms produce stones well beyond that threshold. A baseball-sized hailstone is around 2.75 inches and delivers substantially more energy than the Class 4 test standard accounts for. No standard residential roofing material is designed to survive repeated impacts from stones that large.
If you live in a hail-prone area, the practical implication is that impact-rated roofing reduces damage in moderate hail events but cannot make a roof hail-proof. Insurance companies in many states offer premium discounts for Class 3 or Class 4 roofing, and the investment tends to pay for itself over the life of the roof in areas that see frequent hail between one and two inches in diameter. For the rare event producing truly giant hail, even the best residential roof is going to take damage.
Hail, Graupel, and Sleet
People frequently confuse hail with other forms of frozen precipitation, and the distinctions are worth knowing because they signal completely different atmospheric processes. Hail forms inside convective thunderstorms through the growth mechanism described above, requiring strong updrafts and supercooled water. It falls during warm-season severe weather, often on summer afternoons.
Graupel forms when supercooled cloud droplets freeze onto a snowflake, coating it in a layer of rime ice. The result is a small, soft, opaque pellet that crumbles easily when squeezed. Graupel does not require a powerful updraft and can fall from relatively modest winter or spring clouds. It looks like tiny styrofoam balls and is usually harmless.
Sleet, on the other hand, is not a convective phenomenon at all. It forms when rain or partially melted snow falls through a layer of subfreezing air near the surface and refreezes into small, hard, clear ice pellets. Sleet is a winter weather event associated with temperature inversions, not thunderstorms. If you hear ice pellets bouncing off your window during a cold rain, that is sleet.
The size alone can sometimes tell you what you are dealing with. Hailstones can range from 5 millimeters to well over 10 centimeters. Graupel pellets are typically 2 to 5 millimeters. Sleet pellets are usually smaller than 5 millimeters and distinctly translucent. But the real diagnostic is the weather: if it is a summer thunderstorm, the ice is hail. If it is a winter weather system with mixed precipitation, it is sleet or graupel.
Why Some Hail Is Spiky and Some Is Round
Not all hailstones are spheres. Some arrive as lumpy, irregular masses with protruding lobes and spikes, while others are nearly perfectly round. The shape depends largely on which growth mode dominated during the stone’s time in the cloud and how it was oriented as it fell through the growth zone.
Stones that grew predominantly under wet growth conditions tend to be smoother and more spherical. The liquid film on the surface allows water to flow and redistribute before freezing, filling in irregularities. Stones that grew mostly through dry growth are more likely to be rough and irregular, because the rapid freezing locks in whatever shape the individual droplet collisions create.
Tumbling also matters. A stone that rotates freely as it grows tends to accumulate ice more evenly around its surface, producing a rounder shape. A stone that falls with a preferred orientation, one flat side always facing down, can develop a conical or disc-like shape because the leading face collects more supercooled water than the trailing side. Very large hailstones often have lobed shapes because they grew by absorbing smaller hailstones or large frozen drops, which stick to the surface as bulges rather than smoothly merging into the existing ice.
You can sometimes read a hailstone’s growth history from its shape. A smooth, round stone with clear ice on the outside likely spent its final growth phase in wet conditions. A lumpy, opaque stone with visible protrusions grew in drier, colder conditions. The most visually dramatic stones, with their spikes and irregular lobes, usually had a chaotic growth environment, tumbling unpredictably through varying conditions and collecting ice unevenly.