On 30 August 2022, giant hailstones pelted the Catalonia region of northern Spain. Seventy people were injured, and one toddler was killed as the solid balls of ice 10–12 centimeters in diameter fell from the sky during a severe thunderstorm. Though hailstorms are not uncommon in Catalonia, the stones produced in this event were the largest ever recorded in the region.
Hail size is described by the National Weather Service on the basis of a comparative diameter to familiar objects—hailstones can be the size of a pea, a nickel, or a ping-pong ball, for instance. During the Catalonian storm in 2022, hailstones were softball sized.
“We just don’t know a lot about hail—especially giant hailstones bigger than 6–7 inches in diameter. They’re very rare, so it’s hard to study them,” said Victor Gensini, a meteorologist at Northern Illinois University. “The more extreme or rare an event is, the less likely you are to have observed it.”
Luckily, residents in Catalonia collected many giant hailstones from the exceptional event and stored them in their freezers.
In a recent study published in Frontiers in Environmental Science, researchers from the Meteorological Service of Catalonia and the University of Barcelona revisited the preserved hailstones. By using computed tomography (CT) scanning equipment from a dentist’s office, the researchers created never-before-seen images of the inner structure of giant hailstones. This type of imaging may provide more information about how large hail forms, eventually helping scientists and meteorologists better understand how this type of severe weather event happens.
From Cloud to Ground
During a thunderstorm, strong updrafts pull rain into the cold upper atmosphere, where it freezes. These supercooled water droplets are blown aloft again and again by the updrafts—each time, they collect more layers of frozen moisture, like a jawbreaker candy, explained meteorologist Katie Nickolaou from WLNS in Michigan, who was not involved in the study.
“The hail just keeps doing this bobbing effect until it becomes so heavy the wind can’t possibly hold it up, and it comes crashing down to Earth,” Nickolaou said.
“To get massive hail, you want to have wind speeds that are just roaring in that cloud.”
What hailstones look like can depend on the temperature and the amount of liquid available inside the cloud. During wet growth, a hailstone nucleus collects supercooled liquid droplets that freeze slowly and allow air bubbles to escape, creating a smooth, clear layer of ice. Dry growth happens when the surrounding air temperature is below freezing and the water droplets freeze in place on the nucleus, creating cloudy ice.
The speed of the thundercloud’s updraft (driven by convective energy) is another factor contributing to different sizes and shapes of hailstones, Nickolaou said. Stronger updrafts and related wind speed can keep hailstones suspended in the cloud for longer periods of time, allowing the hailstones to grow. “If you want to have grapefruit-sized hail, you’re going to have updrafts of 98 miles per hour. To get massive hail, you want to have wind speeds that are just roaring in that cloud.”
CT Scans for Teeth and Hailstones
“When I saw it: Wow! You can see the middle of the stone. You can see the embryo that the hail grew from. And you can see how many layers there are.”
After the 2022 storm, researchers from the Meteorological Service of Catalonia obtained 14 hailstones from residents’ freezers and measured the stones’ diameters, weights, and thicknesses, then categorized them as spherical or nonspherical. The heaviest stone weighed 143 grams, about the weight of an apple.
“Previously, when people wanted to see the inner structure of a hailstone, they had to break the stone,” said study coauthor Tomeu Rigo, a meteorologist. Other methods of studying hailstones, like 3D laser scanning or electron microscopy, provide information about exterior shape and chemical composition but don’t offer much information on a hailstone’s growth process.
Instead, meteorologist and the study’s lead author Carme Farnell Barqué talked with an orthodontist friend, who suggested examining the hailstones via a CT scanner.
Similar to the way that a dentist views 3D X-ray images of patients’ teeth, the CT scan gave researchers a 3D view of a hailstone, divided into 512 horizontal slices, each 0.16 millimeter thick.
“When I saw it: Wow!” Farnell Barqué said. “You can see the middle of the stone. You can see the embryo that the hail grew from. And you can see how many layers there are.”
Those layers, too, provide information about the thermodynamics of the cloud from which the hail originated and how wet or dry the conditions were—clues found in the sizes and shapes of the air bubbles present in each sample as well as in the density differences between clear (higher density because of greater water content) and opaque (lower density because of greater air content) ice layers in each sample stone.
Another key finding was that a hailstone’s outward spherical shape may belie a more complicated inner structure. “You can see when you have the hail on your hand, the stone is spherical. But when you see the interior—no, it’s not spherical. It’s conical,” Farnell Barqué said. The exterior shapes of the stones themselves also provided important details about how the stones developed or fell from the sky. Thicker patches on a stone indicated the side facing downward as it fell.
Growing Awareness of Giant Hail
The researchers have many next steps for this technology in mind, including chemical and temperature analysis of each icy layer.
“We have a lot of different shapes and sizes of stone, and we want to compare five or six samples of each to understand better if the growing process is the same,” Rigo said. “We can compare the place where the storm started and the possible conditions which helped the hail to grow very large.” All of these future studies could contribute to building a better forecast—especially for regions that are prone to extra-large hail.
“Typically, when we think of severe weather, we think of hurricanes and tornadoes, but hail is the most costly thunderstorm peril globally by far,” said Gensini, who was not involved in the study. “Anytime researchers can come up with new and innovative ways to study hailstones, it’s usually a great thing for science.”
—Rebecca Owen (@beccapox), Science Writer