The Physics Behind Unusual Branching Hailstones Documented in Illinois

Storm chasers in Illinois document rare star-shaped hailstones, revealing a little-studied ice crystal formation process

A storm chaser in Illinois captured striking star-shaped hailstones during a June 2026 thunderstorm, a phenomenon meteorologists say results from an unusual interaction between supercooled water droplets and ice nuclei in rapidly shifting updrafts. The discovery, documented by a researcher affiliated with the University of Chicago’s atmospheric science program, provides new insights into how extreme turbulence can alter hail growth patterns.

What caused the star-shaped hailstones?

According to Dr. Elena Vasquez, a research associate at the university’s Department of Geophysical Sciences, the formation likely stems from a process called dendritic ice growth—where ice crystals branch out in fractal-like patterns when exposed to specific temperature and humidity gradients. "In most hailstones, the internal structure is concentric layers of ice," Vasquez told News Directory 3. "But when updrafts exceed 100 mph and supercooled water freezes instantaneously around a nucleus, the ice can crystallize in these starburst shapes."

The Illinois hailstones, measuring between 2 and 3 inches in diameter, were collected near Champaign-Urbana on June 26 after a severe thunderstorm system moved through central Illinois. Vasquez’s team analyzed the specimens using X-ray microtomography, revealing intricate internal branching—unlike the typical smooth or layered hailstones documented in prior studies.

How common is this phenomenon?

While star-shaped hail is not unheard of—similar cases have been recorded in Colorado and Wyoming—Vasquez emphasized that the Illinois specimens exhibit a higher degree of symmetry and complexity. "The Champaign storm’s updrafts may have been unusually strong or sustained," she said. "This suggests that such formations could be more widespread than previously thought, but only under very specific atmospheric conditions."

The National Weather Service’s Chicago office confirmed the storm’s intensity, noting wind gusts of 78 mph and hail reports up to 2.5 inches in diameter. However, the star-shaped specimens were isolated to a single storm cell, according to Vasquez’s preliminary findings.

Why does this matter for weather science?

The discovery challenges long-held assumptions about hailstone formation, particularly how turbulence influences ice crystal growth. "Most models assume hailstones grow in a predictable, layered manner," Vasquez said. "But these star-shaped examples show that extreme conditions can lead to entirely different structures."

Researchers are now comparing the Illinois hailstones to archived specimens from the 2010 Denver hailstorm, which also produced unusual branching patterns. The University of Chicago team plans to publish their findings in Journal of Geophysical Research: Atmospheres later this year, with peer-reviewed analysis expected by autumn.

What happens next for storm chasers and meteorologists?

Storm chasers in the Midwest are already scanning radar for similar anomalies, while the National Severe Storms Laboratory is reviewing satellite data to identify potential atmospheric triggers. Vasquez cautioned that the phenomenon remains rare, but advises chasers to collect hailstones immediately after storms—before they melt—to preserve structural integrity for analysis.

For residents in hail-prone regions, the discovery offers a reminder of how quickly storm conditions can shift. The National Weather Service advises checking forecasts for severe thunderstorm warnings, particularly in areas with rapid temperature changes.

Sources: University of Chicago Department of Geophysical Sciences; National Weather Service Chicago office; preliminary research paper submitted to Journal of Geophysical Research: Atmospheres, June 2026.