It is late afternoon in July on the plains of Nebraska. The air has been thick and warm since noon, the kind of heat that makes the cornfields shimmer. Out to the west, a cloud has been building for an hour, its top flattening into a broad anvil against the deep blue. Then the first stones strike the truck roof. A few at first, then a roar like gravel poured from a chute.

Ice is falling from the sky on the hottest day of the week. It seems like a contradiction, but it is one of the most elegant machines in the atmosphere. Inside that towering cloud, air rushes upward fast enough to hold ice aloft, feeding it and building it, until the stone grows too heavy to stay.

The secret begins with a fact that surprises most people. Even on a scorching summer day, the upper parts of a thunderstorm are bitterly cold. Air cools as it rises, by roughly 6 to 10 degrees Celsius for every kilometre. A few kilometres up, temperatures drop below freezing, and near the top of a tall storm they can reach minus 40 or colder. A single cloud can stretch from a sweltering parking lot to a place colder than an Arctic winter.

Within that cloud float countless droplets of supercooled water. This is liquid that has cooled below freezing but has not yet turned to ice, because it has nothing to freeze onto. When a tiny ice pellet, called an embryo, drifts through them, the droplets freeze on contact and coat the pellet like paint on a dipped brush. The storm's updraft, a column of rising air that can exceed 100 kilometres per hour, keeps the pellet suspended in this growth zone.

The classic picture shows hailstones bouncing up and down like yo-yos, making trip after trip. Modern radar studies suggest the reality is often simpler. Many stones ride one or two long, curving paths through the storm, drifting in and out of the strongest rising air. Either way, the principle is the same. Every minute aloft is a minute of growth, and the strongest updrafts allow the longest stays.

Takeaway

Hail is less about cold than about time. A stone grows only as long as the updraft can keep it from falling.

Slice a large hailstone in half and you will often find a pattern of concentric rings, alternating between clear and cloudy ice. Like the growth rings in a tree trunk, these layers record the stone's history. Scientists have studied them for decades, reading each band as a clue to the conditions the stone passed through.

The difference comes down to how fast the water freezes. Some parts of the cloud hold plenty of supercooled droplets and sit only slightly below freezing. There, water spreads across the stone as a thin film and freezes slowly. Air bubbles have time to escape, which leaves clear, glassy ice. Meteorologists call this wet growth. In colder parts of the cloud with fewer droplets, each droplet freezes almost instantly and traps tiny bubbles of air. That produces milky, opaque ice, known as dry growth.

It is tempting to count the rings and conclude that a stone made that many trips, but the layers do not map so neatly. A stone can switch between clear and cloudy ice simply by drifting sideways into a wetter or colder pocket of cloud. What the rings really record is the changing weather inside the storm. They form a diary of temperature and moisture written in ice, much as rock strata record ancient seas and deserts.

Takeaway

A hailstone's rings record conditions, not a count of trips. Layered things, whether ice or rock, tell us about the environments that made them.

Every hailstone is caught in a contest between two forces: the updraft pushing it up and gravity pulling it down. As the stone grows, its weight increases faster than the air resistance holding it up, so it falls faster and faster. Once its falling speed exceeds the speed of the rising air, the updraft can no longer support it. The stone drops, and its growth is over.

That is why the biggest hail comes from supercells, rotating thunderstorms whose updrafts can top 160 kilometres per hour. A golf-ball-sized stone needs rising air of roughly 90 kilometres per hour to stay aloft, and softball-sized hail needs far more. The current U.S. record hailstone fell near Vivian, South Dakota, in July 2010. It measured 20 centimetres across and weighed nearly a kilogram. Much larger stones would demand updrafts that few storms can sustain.

The size of hail at the ground also depends on the journey down. A hailstone melts as it falls through warm air, and small stones sometimes turn to rain before landing. This helps explain why the high plains from Colorado to Nebraska earn the nickname Hail Alley. Their high elevation leaves less warm air beneath the cloud, so stones arrive larger. Tropical storms, despite their power, often lose much of their hail on the long fall through humid heat.

Takeaway

Every hailstone reflects a balance between lift and weight. What reaches the ground depends as much on the fall as on the growth.

The next time a summer storm rattles your roof with ice, you will be hearing the end of a story that began kilometres overhead. It started in air colder than a freezer, held up by wind faster than highway traffic.

That understanding has practical uses. A tall, anvil-topped cloud on a hot afternoon signals a powerful updraft, and powerful updrafts mean hail is possible. When one looms, move your car under cover and get indoors.