Most hailstorms produce relatively small pellets. But sometimes they can produce hailstones two-thirds the size of an AFL football.
The American Great Plains, for example, have produced some true monsters, with the U.S. recordholder, from a field near Vivian, South Dakota, measuring 20 centimetres across and weighing in at 879 grams.
Hailstones nearly that large have also fallen in Kansas and Nebraska, and it’s not just an American phenomenon. Huge hailstones have fallen in such widely scattered locations such as Argentina, Bangladesh, Libya, and various parts of Europe. The Australian recordholder fell last October in Queensland and measured 16 centimetres across.
Gigantic hailstones are rare enough that most of us will never see one. Which, as Ian Giammanco of the Insurance Institute for Business & Home Safety told a recent workshop on hail and hailstorms in Boulder, Colorado, is a good thing.
Normally, Giammanco said, hail rarely causes severe injuries. “But if you find yourself caught in a hailstorm that has stones like these, that’s absolutely a threat to your life.”
In fact, only a few weeks ago, hail measuring up to 11 centimetres across pelted a town in Spain, injuring 50 people and killing a toddler.
Giant hailstones are also fascinating to scientists. “In a hailstorm, only a few select hailstones become the largest,” says Andrew Heymsfield, a hail researcher at the National Center for Atmospheric Research, who also spoke at the Colorado workshop. “What are the underlying physical processes that select these ‘favoured’ hailstones?”
The traditional model is that hailstones start as small ice particles that get wafted aloft by updrafts within the thunderstorm, growing ever larger until they move out of the updraft and fall to the ground.
“There’s this simple model that it goes up in the updraft, maybe moves around a few times, then falls out,” said a third speaker at the workshop, Rebecca Adams-Selin of Verisk Atmospheric and Environmental Research.
But, she says, that’s not how it actually happens. “We have fairly sophisticated computer models now, and can simulate hail moving through a storm.”
These models show that when trajectories of thousands of incipient hailstones are plotted in three dimensions, they follow a multitude of trajectories. “There are all sorts of small-scale processes going on,” she says—a possible explanation for why a single hailstorm can produce hailstones with a wide range of sizes.
Meanwhile, Heymsfield’s team has been using 3D printers to create models of giant hailstones like the one collected in Vivian, South Dakota. These model hailstones were then put in a vertical wind tunnel to examine how they fell. The goal was to determine how hard they would hit, but one of the most interesting findings, Heymsfield says, was that the highly irregular shapes of these super-large hailstones had a major effect on their flight.
These hailstones often have strange-looking lobes that make them look more like pinwheels than the spherical shapes normally associated with hailstones. “They didn’t tumble the way we thought [they would],” Heymsfield says. “The lobes almost act like helicopter rotors. They generate lift, which enables the stone to stay aloft longer and get to these big sizes.”
Heymsfield’s and Adams-Selin’s work is part of a much larger initiative to learn more about hail and hailstorms, on which research has been lagging for many years.
The workshop at which they spoke was the Second North American Workshop on Hail & Hailstorms. The first was in 2020. Prior to that, you have to look back to the 1970s, when the South Dakota School of Mines and Technology, in consort with the U.S. National Science Foundation, flew a specially reinforced T-28 two-seater prop plane through hailstones. “It was like a Brinks armour-plated truck, flying in the air,” Heymsfield says.
The plane was retired in 2003, however, and much of its data is so old that it is no longer available to modern computers.
On the ground, there’s even less data. “There hasn’t been a major U.S. field program to study hail for the past 40 years,” Heymsfield said.
Efforts to remedy this begin with a program called ICECHIP (In-situ Collaborative Experiment for Collection of Hail in the Plains), which, if approved for funding, will run in June 2024 and May 2025. “It would in essence be us, chasing hailstorms,” Adams-Selin says.
Goals range from collecting hailstones and tabulating statistics on their sizes, to seeing where in the thunderstorm they fell. There are also plans to set out sensor mats in the path of the storm in order to measure the impact with which each stone falls, and to monitor the whole process with mobile radar instruments.
It’s even possible to dissect the stones to observe the ringlike layers that indicate how they grew, and to take samples from each layer for trace isotope analysis to determine the precise conditions under which they formed. From that, Adams-Selin says, “you can back out information about how the hail has moved through the storm.”
One goal is better forecasting, or at least the ability to give better short-term warnings so that people like those injured and killed in Spain have time to take cover. But it’s also useful for understanding how factors like climate change and urban growth might be setting us up for ever-more-damaging hailstorms.
That’s important Giammanco says, because however much attention tornadoes, wind, and lightning get, the vast majority of the damage done by severe thunderstorms (about 70 percent) comes from hail.
Last year in North America alone, he says, hail produced more than US$16 billion in damage, with the potential of $3 to $4 billion in damage from a single hailstorm if it hits a major city. And it’s not just a North American problem. There have been several billion-dollar hail events in Australia in the past few years, Adams-Selin says.
Climate change may be exacerbating this. That’s partly because it may be shifting severe thunderstorms into regions not previously accustomed to dealing with them. “I think we are seeing some geographic shifts,” Giammanco says.
But a warming climate is also changing hail-producing processes, Heymsfield says, though in this case the impact isn’t as clear. On the one hand, warmer temperatures increase the amount of moisture in the air, potentially fuelling the growth of large hail. But they also elevate the “melting layer”—the height at which falling hail begins to melt. The result, Heymsfield says, is that in areas where it’s really warm at the surface, there may be less hail—or at least less small hail—because it melts before it gets there.
Meanwhile, much of the research depends on people, like the resident of Vivian, South Dakota, who collected and preserved America’s 20cm hailstone. Without him, nobody would ever have known it existed.
So, what do you do if you find yourself in a similar position?
“Put it in a freezer bag,” Giammanco says. “Don’t hold it in your hand too long. Send a note to your local broadcast meteorologist or the National Weather Service.”
And above all: “Don’t put it in a margarita.”