Watch out: Hail can get really big!

Chunks of ice fall from the sky, bounce off hard surfaces — and sometimes even break windshields and hurt crops. Maybe you’ve seen these pieces of ice laying on the ground after a storm. Their pummeling barrage starts and stops suddenly, seemingly without warning. This is a hailstorm. And it can be a truly impressive spectacle.

Hail forms high in the clouds during a storm. There, some raindrops freeze solid. As more and more raindrops collide, an icy “stone” grows in size until it’s too heavy to remain aloft.

Particularly large or dense hailstones can damage human-made structures and objects, such as buildings and cars. Zach Lebo knows this firsthand. A meteorologist, he works at the University of Oklahoma in Norman. His parents in Pennsylvania had to replace their home’s roof twice in the last 10 years due to hail damage.

Researchers are hoping to better understand some of the mysteries of hail. One big question is how hail moves through the atmosphere and builds up in size. Some hailstones have measured more than 13 centimeters (5 inches) across. That’s larger than a softball!

Scientists used to think that hailstones grew by taking a looping path through the atmosphere. Think of their path as upward and downward like a merry-go-round tipped on its side. But such looping motions are actually rare, scientists in China have now shown. They analyzed hailstones collected by citizen scientists.

Since 2016, people had been collecting hailstones across 15 provinces in China. These citizen scientists gathered more than 3,000 of these bits of ice and stored them until people on Qinghong Zhang’s team could retrieve the hail. An atmospheric scientist, Zhang works at Peking University in Beijing. The best place for citizen scientists to store hail is, not surprisingly, a cold place, she says. “They put the hailstone in their freezer at home.”

Her group analyzed the chemistry of some of that hail. First, they sliced the stones, which allowed them to see layers of ice inside. The researchers then analyzed the specific isotopes of oxygen present in the frozen water within each layer. By matching that chemistry with the properties of the atmosphere at different heights, they could estimate the paths each ice ball had taken through the sky.

Only 1 out of the 27 analyzed hailstones appeared to have moved like a tipped-over merry-go-ground. That was unexpected, says Zhang. “It’s surprising.”

ICECHIP may answer some mysteries

Studying the shapes of hailstones also can help reveal how each hailstone forms. Hail is seldom perfectly spherical, says Becky Adams-Selin. She’s a hail scientist who lives in Omaha, Nebraska, and works at Atmospheric and Environmental Research, an environmental consulting company. Some hailstones sport lobes, she notes, and others can have lumps: “You can get all sorts of weird shapes.”

Over the spring and summer of 2025, Adams-Selin has been part of a team collecting hail samples from across the United States. The researchers have traveled across the Front Range of the Rocky Mountains and the Central Plains. This area includes Montana, Wyoming, Colorado, the Dakotas, Nebraska, Kansas, Oklahoma, New Mexico and Texas. And it’s known for producing big storms that regularly spew hail. The project is called ICECHIP (short for In-Situ Collaborative Experiment for the Collection of Hail in the Plains).

ICECHIP instruments measure things such as the air temperature, rainfall and the force with which hail strikes the ground. Adams-Selin also developed a device to store hailstones as the team drives around. “It’s like a giant funnel that collects hail and funnels it into a battery-powered freezer,” she explains, until they get back to the lab.

Hailstones are a window into the atmosphere, says Liye Li. She’s a meteorologist with the World Meteorological Organization in Geneva, Switzerland. It can be hard to directly observe where hail forms, she explains. “We’re blind to what’s happening inside the clouds.” But hailstones deliver clues about what’s going on high in the sky right into scientists’ hands.