Hail Doesn’t Form the Way You Think, Scientists Say

In high school Earth science, you likely learned that hailstorms form when strong upward winds push water droplets to freezing altitudes, where they solidify and gather additional layers of ice as they cycle up and down the storm cloud. When they grow too heavy, they plummet to the ground. For once, however, new research suggests that the truth might be simpler than what we were taught to believe.

An international team of researchers has shed light on the formation and growth of hailstones in storm clouds by analyzing chemical signatures within the ice. Their analysis, published April 2 in the journal Advances in Atmospheric Sciences, shows that most hailstones form through simpler, more direct paths than previously assumed—an observation that challenges the idea that most hailstones form by repeatedly cycling through storms clouds. In addition to contesting widespread notions about hail formation, the study could help improve forecasts of severe weather.

“This work fundamentally changes how we understand hail formation,” Qinghong Zhang, a lead researcher of the study from Peking University’s Department of Atmospheric and Oceanic Sciences, explained in a statement. “By moving beyond assumptions to actual chemical evidence, we’re building a more accurate picture of these destructive weather phenomena.”

The researchers from China and the U.S. analyzed stable isotopes (non-radioactive forms of atoms) to reveal the altitudes and temperatures at which the layers of 27 hailstones from China formed—essentially, a vertical map of the hailstones’ movement in storm clouds.

The analysis revealed that most followed relatively simple paths. Ten hailstones grew while descending, 13 were elevated by upwinds only once, and three moved mostly horizontally. Only one of the 27 hailstones showed evidence of consistent cycling in storm clouds, challenging long-held beliefs about the prevalence of this growth process.

Furthermore, “trajectory analysis revealed that similar-sized hailstones from a single storm tended to form at similar heights, whereas those larger than 25 mm [0.98 inches] in diameter exhibited at least one period of upward growth,” the researchers explained in the study. This is consistent with the observation that intense hailstorms are associated with strong updrafts, or upward-moving currents of air.

Although most hailstone growth takes place at temperatures between 14 and -22 degrees Fahrenheit (-10 and -30 degrees Celsius), the researchers also discovered that hail “embryos” can develop within a broader range: 16 to -28 degrees Fahrenheit (-8.7 to -33.4 degrees Celsius).

The 27 studied hailstones were collected by citizen scientists in China through a project coordinated by the World Meteorological Organization. Moving forward, Zhang and her colleagues aim to study more hailstorms and integrate the analysis of particulate matter (a mix of liquid and solid particles) found in hailstones.

Ultimately, the study refines our understanding of hailstone formation, with important implications for forecasting severe hailstorms—especially as some scientists warn that global warming may promote the development of larger hailstones.