CT scans have revealed new details about the internal structure of hailstones, offering valuable insights that could improve our understanding and forecasting of hail formation. Hailstones form during thunderstorms when raindrops are pushed into the colder upper regions of a cloud, where they freeze. As these particles grow heavier, gravity pulls them down, and they continue to increase in size as they fall, sometimes causing significant damage to property and posing a danger to people.
Since the 1960s, scientists have studied hailstone growth, but this often involved breaking the stones apart. Recently, researchers in Catalonia employed computed tomography (CT) scans to analyze giant hailstones from a powerful thunderstorm that struck the Iberian Peninsula in the summer of 2022. Their findings, published in Frontiers in Environmental Science, provide new insights into how hailstones form and grow.
The study used CT scans to examine hailstones without damaging them, offering a direct look at their internal structure. Lead author Carme Farnell Barqué, a researcher at the Meteorological Service of Catalonia, explained, "For the first time, we can directly observe the entire internal structure of hailstones, which could help improve hail formation forecasts."
After the storm hit on August 30, 2022, the team tracked its path, collected hailstones from local observers, and analyzed them in the lab. Some of the hailstones were as large as 12 cm in diameter. Using equipment from a dental clinic, the researchers scanned three randomly selected hailstones, uncovering detailed 3D images of their internal layers. "We were amazed by how clear the images turned out," said Prof. Xavier Úbeda, senior author of the study.
The CT scans provided 512 slices of each hailstone, revealing its core and growth layers. These scans revealed irregularities within the stones, even though they appeared to be nearly perfect spheres externally. "We found that the embryo of the stone could be located far from the center, implying that hailstones can grow in three directions unevenly," explained Dr. Tomeu Rigo Ribas, co-author of the study.
Additionally, the researchers observed varying densities in different layers of the hailstones. In two of the samples, the thicker parts of the stones indicated the side that was facing downward during their fall. These findings challenge previous assumptions that large hailstones can only form irregular shapes. The study showed that some stones with regular external shapes grew unevenly inside, while others with irregular shapes had a homogeneous growth pattern.
Although CT scans are costly, the researchers believe that these findings, along with future research, will help mitigate the damage caused by giant hail events, which are expected to become more frequent and impactful in the future. The study's insights into hailstone formation could lead to better predictions and preparations for such severe weather events.
