A scanner at a dentist’s office has produced the first high-resolution 3D images of the internal structure of large hailstones. Such detailed views could help researchers better forecast which storms will generate these destructive pieces of ice.

“The first result was impressive,” says Carme Farnell Barqué at the Meteorological Service of Catalonia in Spain. “Wow! We can see the interior of the stone without breaking it. We could see different layers, with different densities.”

The hailstones fell during an intense storm that struck the north east of Spain in 2022, killing one child, injuring dozens of people and causing millions of dollars’ worth of damage. The largest hailstones that fell had diameters of 12 centimetres, about twice the size of a tennis ball.

A few days after the storm, Farnell Barqué and her colleagues asked around to see if anyone had kept some of the hail. They collected 14 hailstones, up to 8.5 centimetres in diameter, that people had stored in plastic bags in their freezers.

Hailstones form when layers of supercooled water accumulate on an initial embryonic ice particle in a storm. The shape and density of these layers of ice within hailstones can reveal details about the growth process. But normally, researchers can only study a few cross-sections of a single hailstone by slicing open the ice with a hot knife.

In this case, an orthodontist friend of Farnell Barqué suggested the researchers instead use a CT scanner to reveal the hailstones’ complete internal structure. Such a scanner was available in a dentist’s office.

The team scanned three of the hailstones, generating hundreds of cross-sections showing variations in density within each piece of ice. Some of the details were surprising: for instance, although the hailstones were spherical, their nuclei were located far off centre. Farnell Barqué says this suggests the thickest part of the stone formed as it was falling, rather than when it was cycling between different altitudes on updrafts within the storm.

Julian Brimelow at the Northern Hail Project in Canada says a few other small hailstones have been scanned this way, but the stones from Spain are much larger. “This is important, because we are still not certain how and where in a thunderstorm hail grows to achieve such impressive sizes.”

This better understanding could improve forecasts of hail size in future storms. “We can associate each layer of the growing part with the radar data on the evolution of the thunderstorm,” says Tomeu Rigo at the Meteorological Service of Catalonia. “Then it’s possible to relate this with new thunderstorms and project our results to the future.”

“We probably do need to look at this for more hailstones,” says John Allen at Central Michigan University, who is planning a large hailstone-collecting survey in the US Great Plains in 2025. “The question is: how viable is this method for large numbers of stones?”