In August 2022, residents of Catalonia were surprised by a rain of unusually large icy projectiles that bombarded northeastern Spain, causing physical injuries to dozens of individuals and widespread property damage.
Samples collected in the aftermath of the incident, recognized as the most severe event involving a hail-bearing supercell ever recorded in the region, are now offering clues toward understanding how hailstone formation of such extreme magnitude can occur.
The findings, detailed in a new study published in Frontiers in Environmental Science, reveals the notable first successful examination of the internal structure of hailstones without their destruction, achieved using state-of-the-art computed tomography (CT) analysis.
The researchers’ analysis has revealed high-resolution 3D images of the massive hailstones, providing unprecedented insights into their formation process.
Nature’s Icy Falling Projectiles
Hailstones result from atmospheric conditions where rain drops are carried upward to the extremely cold upper layers of a thunderstorm, allowing them to freeze and accumulate additional layers of ice. As they grow with the addition of more ice formation on their exterior, these frozen droplets eventually succumb to the effects of gravity, which brings them back to Earth.
The sudden onset of hailstone formation can present several potential dangers, from destruction of property to safety risks and even physical injury. Despite the well-known processes that lead to their formation, in the past, scientists seeking a deeper understanding of the dynamics of hailstone generation were required to break these frozen objects apart to study their internal structure. Doing so naturally imposes limitations on attempts at unraveling deeper insights into the forces of nature that give rise to their formation.
That is, until now. Researchers with the Meteorological Service of Catalonia and the Department of Geography at the University of Barcelona, Spain, now report the first known study of the interior of whole preserved hailstones using a novel CT scanning approach.
“We show that the CT scanning technique enables the observation of the internal structure of hailstones without breaking the samples,” said Carme Farnell Barqué, lead researcher from the Meteorological Service of Catalonia, in a statement.
“It is the first time that we have a direct observation of the entire internal structure of hailstones, which can provide clues to improve hail formation forecasting,” Farnell Barqué said.
In the aftermath of the storm two summers ago, Farnell Barqué and his colleagues retraced the storm’s path by collecting eyewitness accounts of its arrival. During their inquiries, the scientists were also able to collect samples of the massive hailstones—some of the largest presently known—that local Catalonians had preserved in their freezers.
Employing CT scanning capabilities, Farnell Barqué and the research team produced 512 internal computer-generated “slices” of each stone, revealing distinct layers and cores.
“We wanted to use a technique that would provide more information regarding the internal layers of the hailstones, but without breaking the samples,” said senior researcher Prof. Xavier Úbeda from the University of Barcelona.
However, the level of success achieved by the team’s investigation surprised them, as well as what they discovered inside the giant hailstones.
“We didn’t expect to obtain as clear imagery as we got,” Úbeda said.
Surprising Discoveries Within Giant Hailstones
Among the unique features revealed in the team’s scans were irregular internal axes, which appeared even when the hailstones had a spherical external appearance. Some of the samples also revealed an internal icy core located a significant distance from the actual geometric center of the hailstones, which points to uneven growth processes.
The team also uncovered evidence that different ice layers displayed different densities, with thicker portions of the hailstones likely forming on the side facing downward during descent.
“Until now, it was believed that very large hailstones could only have irregular shapes,” Barqué explained. “However, we observed that the external and internal shapes can differ significantly.”
Dr. Tomeu Rigo Ribas, from the Meteorological Service of Catalonia and one of the new paper’s co-authors, highlighted the significance of these findings, noting that some of the hailstones appear to have grown in diverse ways, sometimes showing regions of growth in more than one or two directions.
“We show that the embryo can be located far from the center,” Ribas said. “This fact implies that the stones can grow heterogeneously in three directions.”
Although the team’s recent studies of the massive Catalonian hailstones offer unprecedented new insights into the formation of these potentially dangerous chunks of falling ice, the CT scanning methods they used still present a few challenges. Among these are the high costs associated with the scans and their occasional tendency to produce imaging anomalies.
Still, the team believes their findings offer significant new insights into hailstone formation, particularly as extreme weather events become more frequent and potentially destructive.
“We hope that our work can provide new information that may help mitigate damage to society,” Farnell Barqué said.
The team has detailed their findings in a new paper, “Internal structure of giant hail in a catastrophic event in Catalonia (NE Iberian Peninsula),” published in Frontiers in Environmental Science.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.