Why Does Hail Form in Warm-Weather Storms?

Ice hammering down on a warm afternoon feels like a mistake in the weather. The air at street level may be sticky and hot, the sky only minutes earlier a bright summer blue, yet the stones bouncing off car roofs are solid ice. The paradox dissolves once you look at the storm from the side rather than from the ground: a summer thunderstorm is a tower, and its top stands in a different season from its base. Hail does not fall in spite of warm weather. It falls because of it. Heat and humidity near the ground are the fuel that builds the kind of storm strong enough to manufacture ice and hurl it earthward.

Why does a hot day build a stronger storm?

A thunderstorm begins with warm, humid air at the surface and an unstable atmosphere above it. When that surface air is lifted, by a front, by terrain, or simply by daytime heating, it cools as it rises, and the water vapour it carries condenses. Condensation releases latent heat, which keeps the rising parcel warmer than the air around it, so it keeps accelerating upward. That rising current is the updraft, and in a vigorous summer storm it can race upward at highway speeds and beyond. This is the first half of the answer. A cold winter day rarely produces hail, not because winter lacks ice, but because winter air near the ground holds little moisture and generates little buoyancy. Summer air is loaded with both. The stronger the surface heating and the more humid the air feeding the storm, the harder the updraft pushes, and the longer and higher it can carry its cargo. Temperature also falls with height in the lower atmosphere, so a cloud tall enough reaches air well below freezing even in July. Inside a mature thundercloud, the layer where temperatures run between freezing and about minus 40 degrees Celsius is full of droplets that are liquid yet colder than the freezing point. Meteorologists call this supercooled water, and it is the raw material of hail.

How does a raindrop turn into a hailstone?

High in the storm, an updraft sweeps ordinary raindrops upward into that freezing zone, where they freeze into small ice pellets. Each pellet then grows by colliding with supercooled droplets, which freeze onto its surface on contact. NOAA describes two ways this growth happens, and the difference is visible in a sliced hailstone. When the surrounding air is only just below freezing, the collected water spreads across the tumbling stone and freezes slowly, letting trapped air bubbles escape and leaving a layer of clear ice. When the air is much colder, each droplet freezes instantly on impact, trapping its air bubbles in place and leaving a layer of cloudy, white ice. So a hailstone is not a frozen raindrop that simply got bigger. It is an ice nucleus wrapped in successive coats, each coat recording the temperature and water content of the part of the cloud it passed through. A stone that drifts through different pockets of the storm picks up alternating clear and cloudy bands, like rings that record a journey rather than a calendar. It is tempting to picture the stone bobbing up and down through the cloud like a yo-yo, gaining one layer per trip. The National Severe Storms Laboratory warns against that tidy image. The winds inside a storm are not simply vertical: horizontal winds, including the rotating flow of a supercell, sweep stones sideways across and near the updraft, so conditions change even without repeated up-and-down cycles. And stones do not grow by being carried to the very top of the storm, because at extreme heights, below about minus 40 degrees Fahrenheit, all the water has already frozen solid, leaving no liquid droplets for the stone to collect.

When does the stone finally fall?

A hailstone stays aloft only while the updraft beneath it is strong enough to support its weight. As it grows heavier, or as the updraft weakens, gravity wins and the stone drops. NOAA's updraft table makes the relationship concrete: holding up a pea-sized stone takes an updraft of about 24 miles per hour, a golf-ball-sized stone needs roughly 64 miles per hour, and a grapefruit-sized stone demands close to 98 miles per hour. That is why only the most powerful storms, usually supercells with sustained, rotating updrafts, produce the largest hail. Ordinary multi-cell storms, whose mature stage is comparatively brief, tend to make many smaller stones instead. The fall itself is fast and sometimes slanted. Smaller stones can be blown clear of the updraft by horizontal winds, which is why the biggest stones usually land closest to the updraft itself. Wind-driven hail can fall at an angle or nearly sideways, tearing siding, smashing windows, and injuring anyone caught in the open. Recent wind-tunnel work with three-dimensional casts of real hailstones suggests typical fall speeds of 25 to 40 miles per hour for stones one to one and three-quarter inches across, and 44 to 72 miles per hour for stones two to four inches across, with the very largest possibly exceeding 100 miles per hour.

There is one more hurdle between cloud and ground: melting. A stone must survive the warm air below the cloud base. In Florida, which sees more thunderstorms than anywhere else in the country, the freezing level in summer storms sits so high that much hail melts into rain before landing. The states that usually record the most hailstorms are instead Nebraska, Colorado, and Wyoming, where the freezing level is lower and the stones stand a better chance of arriving intact. The corner where those three states meet, nicknamed hail alley, averages seven to nine hail days a year. The same charge separation that makes lightning also depends on this icy interior: colliding ice, hail, and soft hail pellets called graupel are the ingredients that build a storm's electrical charge. Hail and lightning are not separate shows; they are two products of the same violent cloud. Above those same storm tops, brief red flashes known as sprites flicker in the upper atmosphere, another reminder of how much activity a single thunderstorm packs into the sky.

What do people get wrong about summer hail?

The most common mistake is treating hail as a winter phenomenon that has arrived out of season. In reality, the season that matters most is the one at the top of the cloud, not the one on your porch. A hot day with humid air and a trigger to lift it is exactly the recipe for a tall, hard-working storm, and the taller the storm, the colder its summit. A second mistake is assuming that a short, sharp hailstorm means the storm was weak. Hail falls in paths called swaths as the storm moves, and a swath can be as narrow as a few acres or as wide as ten miles and a hundred miles long. Your street may be battered while the next neighbourhood over gets only rain, because the stones fell out where the updraft was strongest at that moment. The record shows how far this process can go. The largest hailstone recovered in the United States fell at Vivian, South Dakota, on June 23, 2010, measuring 8 inches across with a circumference of 18.62 inches and a weight of 1 pound 15 ounces. It took a storm of extraordinary strength to grow and support a stone that size, and a freezing layer low enough for it to reach the ground nearly whole. Hail, then, is summer heat turned against itself. Warm ground air rises, builds a tower into the cold sky, and comes back down as ice. The next time stones rattle across a sun-warmed roof, the surprise is understandable, but the physics is not contradictory at all: the heat made the storm, and the storm made the ice.