Hail isn't the most common weather hazard, but it can be one of nature's most efficient demolition tools. Anyone who's been caught in a hailstorm can tell you that the assault can really hurt. Hail kills an untold number of animals across the world each year and destroys many tonnes of crops. Over 200 people were injured by a 1995 megastorm that struck an outdoor festival in Fort Worth, Texas, with little warning and few places to take cover. In other hail-prone areas, such as China and India, hailstorms have been known to kill more than 100 people. Deaths in the US are rare, but in 1979 a three-month-old Colorado infant died after being struck by an orange-sized hailstone, and in 2000 a Fort Worth man was killed by hail when he tried to move his car out of harms way.

A hailstone's trip to earth

We have thunderstorms to thank, or blame, for hail's existence. It takes the strong updraughts and the cyclonic spin of a severe storm to make big hail. That's why it tends to occur in the same parts of the world that are most prone to severe thunderstorms. Surveys from France, South Africa, Canada and India show that the lion's share of the hailstones they get are less than 2.5cm/1 in wide, but all of these countries have seen hailstones more than three times as large. Smaller hail is far more common, occurring just about anywhere it thunders. Even non-thundery winter storms can be vigorous enough to produce tiny, soft hailstones called graupel.

The world's most infamous hail zones, such as northeast Colorado and the pampas of Argentina, are high-altitude plains in the lee of mountains. When a storm forms over elevated terrain, a bigger
percentage of it extends above the freezing layer, giving hailstones a chance to grow. Hail, in fact, served as one of the most important clues to ancient weather observers like Aristotle that the atmosphere gets colder with height.

A hailstone usually begins as a tiny ice crystal and grows by encountering a special kind of moisture – supercooled water droplets. In strong storms, the air is rising so quickly that some of the inflowing water vapour doesn't have a chance to freeze onto a solid nucleus, like a bit of dust or salt These supercooled droplets can remain liquid even at temperatures well below 0°C/32°F. Suspended in the frigid air, all they're waiting for is a surface on which to freeze – and a growing hailstone does the job nicely.

Many hailstones take only a single trip through a storm. Growing as they ascend, they're buoyed on winds that can blow upward at over 160kph/100mph. Eventually, the hailstone is either flung out of the updraft or becomes heavy enough to fall through it. Updraughts in a hail-producing storm tend to be tilted, so most falling hailstones quickly exit the updraught and descend to earth. However, if you slice open a giant hailstone, you're likely to find evidence of multiple journeys. Most giant stones have an onion-like structure, with perhaps four or more bands of clear, solid ice separated by milky layers. The milkiness is due to tiny air bubbles trapped within and between ice crystals during the freezing process. These layers may be 50 to 90 percent less dense than solid ice.

As best we can tell, giant hailstones get their layers by being recycled. Before they fall out of a storm, they're caught in the inflow and channelled through the updraught for another round trip. It's thought that the alternating milky and smooth layers represent different parts of the storm, where the conditions allow for either slow growth of solid ice or quick, irregular growth of opaque ice. This long-standing concept has been challenged somewhat by recent work that employs computer models to follow a simulated hail-stone through a cloud. Instead of several up-and-down trips, these models point more toward a single, tight helix. In this view, a hailstone circles the updraught as it rises, moving through the same quadrants of the storm with each rotation.

In its final drop to earth, a giant hailstone may glue itself to a few smaller companions, giving it a spiky, irregular shape. Such was the case for the worlds heaviest hailstone, which fell on Coffeyville, Kansas, in 1970. About twenty smaller stones congealed to form the final product It measured roughly 14cm/5.5in in diameter and weighed 759 grams/1.69 pounds. An even larger hailstone fell near Aurora, Nebraska, in 2003: it spanned 17.8cm/7in in diameter, although its weight fell just shy of the Coffeyville record-holder.

“The bigger they are, the harder they fall” doesn't necessarily hold true for hail. Gigantic stones tend to be spun out toward the edge of a storm, where it may scarcely be raining. These grapefruit- or softball-sized missiles are few in number, and they can be separated by 30m/100ft or more. Observers have noted that some of them don't seem to be falling very fast. The layers of low-density ice inside big stones could be making a difference by reducing their downward momentum. Another factor is spin. xMany large hailstones are shaped somewhat like hockey pucks – wider than they are thick – and they may be spinning around their centres at more than forty revolutions a second. One study found that the spin, interacting with wind transitions, could be slowing the stone and curving its path, much like a baseball or cricket ball that's given a twist by the pitcher. The analogy is apt, since hailstones can hit the ground at pitched-ball speeds of more than 144kph/90mph.

On impact

For farmers across the United States, hail is a big consideration. On average, about 2-3 percent of agricultural output across the US is destroyed by hail each year; roughly a quarter of farm output is covered by hail insurance. It's a must in places like the “Hail Alley” of Colorado, Nebraska and Wyoming.

А 1985 hailstorm in Cheyenne (Wyoming's capital and the continent's hail epicentre) left behind icy drifts as high as car windows.

A single misbegotten hailstone can kill a person or destroy a car's windshield, but it takes a lot more hail to ruin a crop. Perhaps surprisingly, it's not gigantic hail that farmers fear most, but the denser, smaller variety. This type tends to fall within heavy rain and can easily blow sideways in a strong wind.

A thick batch of peato walnut-sized hail can virtually destroy a field of wheat or corn in minutes. Some crops are even more vulnerable. Hailstones only 0.5cm/0.2in in diameter can damage the delicate leaves of soybean and tobacco plants. Since heavy hail is produced in a particular part of a storm, it tends to be localized, often falling in streaks as narrow as 0.5km/0.3 miles and only a few miles or kilometres long. The biggest and meanest storms produce hail swaths, multiple streaks that can stretch for over 400km/250 miles.

When hailstorms hit cities, our houses and vehicles (aside from gardens) tend to suffer the most. A round of hail and strong wind pounded Munich, Germany, on July 12, 1984, causing more than $1 billion in damage to some 70,000 homes and 200,000 cars. Similar events destroyed many thousands of vehicles across Denver in 1990 and Sydney in 1999.