Scientists have been studying how hailstones grow since the 1960s but doing so meant breaking them in the process. To better understand the anatomy and growth of hailstones, researchers in Catalonia have used computed tomography (CT) scans to examine the giant hailstones that hit the north-east of the Iberian Peninsula during an exceptionally strong thunderstorm in the summer of 2022.

"We show that the CT scanning technique enables the observation of the internal structure of the hailstones without breaking the samples," said Carme Farnell Barqué, a researcher at the Meteorological Service of Catalonia and lead author of the study published in Frontiers in Environmental Science.

"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."

Storm hunters

After the storm hit Catalonia on August 30, 2022, the researchers tracked the storm's path with the help of local witnesses and collected hailstones from observers who'd saved them in their freezers. Some of the stones measured up to 12cm in diameter.

Back in the lab, three randomly selected hailstones were scanned, using the equipment of a dental clinic.

"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 author Prof Xavier Úbeda, a researcher at the University of Barcelona. "We didn't expect to obtain as clear imagery as we got."