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A team of scientists has cracked open one of meteorology's enduring mysteries—how hailstones grow inside storm clouds—using an innovative approach that analyzes chemical signatures locked in the ice. The findings, published in , challenge long-held assumptions about hail formation and could lead to improved severe weather prediction.

The international research team, led by scientists from Peking University, employed stable isotope analysis to reconstruct the growth histories of 27 hailstones collected from nine storms across China. This technique allowed them to determine the precise altitudes and temperatures at which each layer of the hailstones formed, effectively creating a vertical map of their journeys through storms.

Contrary to the prevailing theory that hailstones grow by repeatedly cycling up and down through storm clouds—creating characteristic alternating layers of clear and opaque ice—the study found most hailstones follow much simpler trajectories. Only one of the 27 specimens showed evidence of true recycling motion. The majority grew either while steadily descending (10 stones) or after a single upward push (13 stones), with three showing primarily horizontal movement.

The research also identified key thresholds for hail growth. While most development occurs in a "sweet spot" between -10°C and -30°C where supercooled water is abundant, the study revealed hail embryos form across a surprisingly wide temperature range from -8.7°C to -33.4°C. Larger hailstones (exceeding 25 mm) require sustained upward motion to remain in optimal growth conditions longer, explaining why severe hailstorms depend on strong updrafts.

"This work fundamentally changes how we understand hail formation," said lead researcher Professor Qinghong Zhang of Peking University. "By moving beyond assumptions to actual chemical evidence, we're building a more accurate picture of these destructive weather phenomena."

The study benefited from an unusual source of data—hailstones collected by citizen scientists across China through a . These publicly contributed samples proved crucial in establishing robust growth patterns across different storm systems.

The international scope now includes cases like Italian hailstorms, where samples were transferred from Europe to U.S. labs before final analysis in China. Such cross-continental collaborations strengthen comparative studies of hail formation mechanisms.

Looking ahead, the team plans to expand their analysis to more storms while combining isotopic data with studies of particulate matter trapped in hailstones. These advancements could lead to better modeling of hail formation and improved severe weather warnings.

The research represents a collaboration between Peking University, Northwest University, the Beijing Weather Modification Office, the Chinese Academy of Meteorological Sciences, and the U.S. National Center for Atmospheric Research.