Hailstones in an egg carton
Gregg Cruger holds hailstones in an egg carton, collected from a storm that damaged his family’s home in Louisville, Colo., on 18 June. Cruger and his wife, Katy Human, witnessed the storm from their front porch. Credit: Helen H. Richardson/Contributor/Denver Post/Getty Images

On 18 June, Katy Human and her family watched as tiny chunks of ice fell from the sky.

At first, Human and her family, of Louisville, Colo., were delighted.

“Hail is cool and relatively rare, so we all went out on the front porch to watch it,” she said. However, “Over the next 10 minutes, it got bigger and bigger.” The family backed up, pressing against the wall of their house as golf- and tennis-ball-sized hailstones pummeled their yard.

Car damage from hail
Leaves and hailstones litter a car that was smashed in a hailstorm on 18 June in Louisville, Colo. Resident Katy Human said that her family’s two vehicles, which were parked outside during the storm, were totaled. Credit: Katy Human

“We have a garage, but both cars were out,” she said. “It went from ‘Oh, my God, isn’t this beautiful’ to ‘Holy crap, this is doing a lot of damage.’”

Human’s family was witness to something that people in the insurance industry have long known. Tornadoes and hurricanes may grab headlines, but when it comes to property damage, the biggest extreme weather culprit is an often overlooked weather phenomenon: hail.

Hailstorm destruction exceeds $10 billion each year across North America, accounting for almost 70% of the property damage in insurance claims from severe storms, said Ian Giammanco, lead research meteorologist at the Insurance Institute for Business and Home Safety’s research center in Richburg, S.C. But, he said, funding for hail studies is limited, and the phenomenon is often treated by the public as a curiosity.

“It’s the Rodney Dangerfield of perils,” he said of hail. “It just doesn’t get any respect.”

“It’s the Rodney Dangerfield of perils,” he said of hail. “It just doesn’t get any respect.”

Giammanco spoke Tuesday on a panel of scientists at the North American Workshop on Hail & Hailstorms. The 3-day symposium, the first of its kind in the United States, was hosted by the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

Making Hail

The workshop occurred just a week after a severe hailstorm pelted Colorado Springs, about an hour’s drive from Boulder, with hailstones reported to be as large as softballs. The storm caused millions of dollars in property damage, injured more than a dozen people, and killed five animals in the Cheyenne Mountain Zoo.

Colorado and the central United States are hot spots for hailstorms, explained panelist Andreas Prein, an NCAR project scientist expert on climate modeling with a focus on severe storms. Colloquially called “hail alley,” the area has the right conditions for severe thunderstorms that can produce large hailstones.

A severe hailstorm needs unstable air with a strong updraft, differing wind speeds and wind directions, air that’s humid close to the ground and dry at higher altitudes, and a freezing point that’s relatively close to the ground.

These conditions work together to form the largest hailstones, by drawing moisture up from the ground and quickly cooling it into hailstones; circulating those hailstones around and around within the storm, allowing them to pick up moisture and grow larger; and giving them a short distance to fall back to Earth, so they don’t melt on the way down, Prein said.

Climate Effects Unclear

Hail damage is expected to increase in coming years, largely driven by population growth and suburban sprawl.

Hail damage is expected to increase in coming years, largely driven by population growth and suburban sprawl. Sprawl means more buildings, and thus “bigger targets for hailstorms to hit,” Giammanco said.

But humans are also thought to be influencing hail itself, through climate change. The effect, however, isn’t straightforward, influencing hail-forming conditions in varied and sometimes contradictory ways, Prein explained.

For example, climate change is expected to increase air buoyancy for strong updrafts, but at the same time it appears to be causing the freezing level to move higher above the ground. “The question is, How are these things interacting, and how are they affecting hail frequency?” Prein said.

He noted that whereas most of the country has seen hail decrease over the past 100 years, certain areas, including the central United States and the mid-Atlantic states, have seen it become more frequent and severe.

“We need more research on that to really understand how climate change and climate variability [are] changing hail hazard,” Prein said.

Efforts to Study Hail

Hail expert and panelist Andrew Heymsfield, NCAR senior scientist and cochairman of the workshop, noted that a hailstorm-penetrating aircraft that had been used to gather important data since the 1970s was decommissioned in 2003, leaving scientists without an important tool for research for the past 15 years.

Another panelist, Kristen Rasmussen, an atmospheric scientist at Colorado State University in Fort Collins, hopes to help reset the scales. She travels to Argentina later this year to study one of the most hail-prone areas in the world.

“We’re trying to study the whole convective process from beginning to end,” Rasmussen said.

Storms producing hail the size of oranges and grapefruits are a yearly occurrence in areas like Mendoza, and hail causes significant monetary damage to the region’s many vineyards. But the area lacked any storm forecasting ground radar or warning systems—such as those provided by the National Weather Service in the United States—until about a year ago, she explained.

For her field project, funded by the National Science Foundation and the U.S. Department of Energy, Rasmussen plans to bring a suite of ground-based radars, hydrological gauges, lightning mapping instruments, and other equipment to learn more about the science of severe storms and hail formation. “We’re trying to study the whole convective process from beginning to end,” she said.

Extensive Damage

For Human, who works with storm researchers as communications director for the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder, the science of hail is now personal.

Still, Human didn’t lose her sense of wonder for hail. “There are these intricately layered concentric circles, like tree rings,” she said. “They’re gorgeous.”

After the 18 June storm, she and her family stumbled out into the street; it was difficult to walk because they kept slipping on the smooth spheres of ice that littered the ground. There they joined their shocked neighbors in inspecting the damage. “Both cars were totaled,” she recalled. Her home’s solar panels were smashed and the roof was battered. Human would later learn that the whole roof had to be replaced. Human estimates the total damages at around $50,000, which will be covered by insurance.

Still, Human didn’t lose her sense of wonder for hail. “They’re beautiful when they crack open,” she said. “There are these intricately layered concentric circles, like tree rings. They’re gorgeous.”

—Ilima Loomis (email: ilima@ilimaloomis.com@iloomis), Freelance Journalist


Loomis, I. (2018), Hail causes the most storm damage costs across North America, Eos, 99, https://doi.org/10.1029/2018EO104487. Published on 16 August 2018.

Text © 2018. The authors. CC BY-NC-ND 3.0
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