Hail is a form of precipitation consisting of solid, hard ice falling from the atmosphere. This surprising weather event is called a hailstorm, and the lumps of ice that fall are known as hailstones. Unlike rain or snow, hailstones form high above the ground through a process involving powerful clouds and strong winds.
What is a Hailstone?
A hailstone is a piece of ice that is at least 5 millimeters (about 0.2 inches) across, which is larger than a small pea. Unlike snowflakes or sleet pellets, hailstones are hard and often have a lumpy or irregular shape. If you cut a large hailstone in half, you might see layers, almost like the rings inside an onion. These layers show how the hailstone grew inside a storm cloud before it finally fell to the ground.
How Hailstones Grow Big
Hail formation begins inside a strong thundercloud packed with liquid water droplets and ice crystals. These clouds contain powerful upward-moving winds called updrafts, which can blow at speeds over 100 miles per hour. A small ice particle gets caught in this updraft and is carried high up where the temperature is far below freezing. There, the ice pellet collides with millions of supercooled water droplets, which remain liquid despite the freezing temperatures.
When these liquid droplets hit the ice pellet, they freeze instantly, adding a new layer of ice in a process called accretion. The updraft holds the hailstone up and carries it through different parts of the cloud. The layers inside a hailstone form because it passes through areas where water freezes quickly, creating a cloudy layer, and areas where it freezes more slowly, creating a clear layer.
The hailstone continues to circle and grow inside the cloud, gaining size and weight. It is trapped in a cycle of being lifted by the updraft and then sinking until the wind catches it again. This process can continue for several minutes, adding layer after layer of ice. The hailstone only falls to the ground when it becomes too heavy for the updraft winds to hold it up any longer.
Comparing Hail Size and Staying Safe
Hailstones come in a wide range of sizes, and scientists compare them to familiar objects to describe their diameter. Small hail can be the size of a pea, while larger pieces might be compared to a quarter, which is about one inch wide. Hail the size of a quarter or larger is considered severe because of the damage it can cause. The biggest hailstones, which can be as large as a baseball or a softball, are only created in the most powerful thunderstorms with the strongest updrafts.
Since large hailstones can fall very fast, sometimes over 40 miles per hour, they pose a danger to people, animals, and property. If you are in a hailstorm, safety is the first priority. You should immediately seek shelter indoors, such as in a house or a sturdy building. Stay away from windows, as the impact of fast-falling hail can cause glass to break. If you are outside and cannot reach a building, use your arms to protect your head and neck while looking for quick cover, like under a bridge or inside a car.