Hail forms when powerful updrafts inside thunderstorms carry raindrops high into the atmosphere, where temperatures are well below freezing. The drops freeze into ice, then grow larger as they collide with more supercooled water on their way back down. The stronger the updraft, the longer the ice stays suspended and the bigger it gets before finally falling to the ground.
How Hailstones Form Step by Step
Every hailstone starts as a small ice particle called an embryo. This can be a frozen raindrop, a clump of snow pellets, or even a speck of dust coated in ice. Thunderstorm updrafts, which are columns of rapidly rising air, push this embryo upward into parts of the cloud where temperatures drop far below 32°F. At these altitudes, liquid water droplets exist in a “supercooled” state, meaning they remain liquid despite being below freezing.
When the embryo collides with these supercooled droplets, the water freezes onto its surface almost instantly, adding a new layer of ice. The updraft then carries the growing hailstone back up again, where it collects still more ice. This cycle can repeat many times. Each pass through the cloud adds another shell of ice, which is why cutting a large hailstone in half often reveals concentric rings, similar to tree rings. Some layers look clear and some look white and opaque, depending on how quickly the water froze and how much air got trapped inside.
Eventually the hailstone becomes too heavy for the updraft to support, and it falls. If the updraft weakens or the stone gets pushed to the edge of the storm where rising air is weaker, it drops out of the cloud and reaches the ground as hail.
Why Updraft Strength Controls Hail Size
The single biggest factor determining how large a hailstone can grow is how fast the air inside the storm is rising. A modest updraft of 25 to 35 mph can keep pea-sized hail aloft. Golf ball-sized hail (about 1.75 inches) requires updrafts approaching 55 to 65 mph. The most extreme hailstones need vertical wind speeds over 100 mph, which only the most powerful supercell thunderstorms can generate.
Supercells are rotating thunderstorms with a persistent, intense updraft. They produce a rain-free zone near their base where the updraft is so strong that precipitation gets swept upward rather than falling. This is where the largest hail grows, cycling through the storm’s core and accumulating ice with each trip. The largest hailstone ever officially recorded fell in Vivian, South Dakota, measuring 8 inches across, more than 18.5 inches around, and weighing nearly 2 pounds.
Conditions That Produce Hail
Not every thunderstorm makes hail. Several atmospheric ingredients have to come together. First, the storm needs strong instability, meaning warm, moist air near the surface and much colder air aloft. This temperature contrast fuels the powerful updrafts that suspend ice. Second, the freezing level can’t be too high or too low. If it’s extremely high (as in deep tropical humidity), raindrops have a long warm journey to the ground and any small hail melts before arriving. If it’s very low, the storm may not have enough warm energy to build strong updrafts in the first place.
Wind shear also plays a key role. When winds change speed or direction at different altitudes, the storm tilts, separating the updraft from the downdraft. This keeps rain from falling through the rising air and choking off the storm, allowing the updraft to persist longer and hailstones to keep growing.
Where and When Hail Is Most Common
In the United States, the central Great Plains see the highest concentration of hail events. The corridor stretching from Texas north through Oklahoma, Kansas, Nebraska, and into South Dakota is sometimes called “Hail Alley.” This region combines all the right ingredients: warm Gulf moisture colliding with cold air from the Rockies, strong wind shear, and frequent supercell development.
Hail season peaks in late spring and early summer, roughly May through July, when surface heating is intense and the jet stream still dips far enough south to create instability. But hail can occur in any month and in any state. Mountain regions like Colorado see frequent small hail from afternoon thunderstorms, while the Southeast gets hail mainly from fast-moving spring storm systems.
How Hail Differs From Sleet
Hail and sleet are both ice that falls from the sky, but they form in completely different ways. Sleet is a winter phenomenon. It happens when snow falls through a warm layer of air, melts into rain, and then refreezes into small ice pellets as it passes through a cold layer near the ground. Sleet particles are tiny, rarely larger than a few millimeters.
Hail forms inside the violent updrafts of a thunderstorm and can grow far larger. By definition, a hailstone must be at least 0.2 inches in diameter. Hailstones start as embryos (which can include sleet-like particles or graupel) and then grow by collecting supercooled water. Sleet falls during cold, layered winter weather. Hail falls during warm-season thunderstorms, often on days that feel hot at ground level.
When Hail Starts Causing Damage
Small hail, anything under about three-quarters of an inch (roughly penny-sized), is mostly a nuisance. It bounces off roofs and windshields without leaving a mark. The damage threshold starts at around 1 inch in diameter, about the size of a quarter. At that size, hail begins to crack and dent standard three-tab asphalt shingles, especially older ones.
Thicker roofing materials hold up longer. Laminated asphalt shingles and cedar shingles typically resist damage until hailstones reach about 1.25 inches. Cedar shakes and fiber-cement tiles can handle stones up to 1.5 inches. Concrete tiles withstand hail up to 1.75 inches, and built-up gravel roofing doesn’t usually show damage until stones reach 2.5 inches. This is one reason the National Weather Service uses 1-inch diameter as the threshold for issuing severe thunderstorm warnings related to hail.
Vehicles are similarly vulnerable. Hailstones around 1 inch can chip paint, while stones 1.5 inches and larger commonly dent sheet metal and shatter windshields. At golf ball size and above, hail can total cars, punch through skylights, and injure people caught outdoors. The costliest U.S. hailstorms cause billions of dollars in property damage in a single event.
How Hail Size Is Reported
Because most people don’t carry rulers outside during a storm, the National Weather Service uses everyday objects as size references. A penny is three-quarters of an inch. A nickel is seven-eighths of an inch. A quarter is 1 inch. From there, the scale moves to larger comparisons: golf balls (1.75 inches), tennis balls (2.5 inches), baseballs (2.75 inches), and softballs (4.5 inches). If you see hail falling, comparing it to a coin or ball gives forecasters useful data for tracking the storm’s severity.