Why was Tuesday's record-breaking hail so huge?

A massive hailstone from Tuesday's storm most likely set a new state record.

Victor Gensini, a meteorologist at Northern Illinois University, was in Kankakee following the supercell storm that caused at least four tornadoes that tore through homes and killed an Indiana couple.

Gensini collected a hailstone and brought it back to his lab at Northern Illinois University, where he studies severe weather and climate. He measured it Thursday afternoon. By his calculations, it's 7.125 inches wide, 16 inches around and weighs just over a pound — 16.5 ounces.

Before Tuesday, the largest hailstone found in Illinois measured 4.75 inches in diameter.

"It didn't just break the record, but it shattered the record," Gensini said. "We've never seen anything like this since we've been recording hail, which started in the mid-1950s."

If, that is, Gensini's measurements are verified by a state meteorologist visiting his lab on Monday.

While massive hailstones are unusual, hailstorms are quite common, Gensini said, especially this time of year,.

Roughly 3,000 to 5,000 hailstorms occur nationwide every year, and "the Chicago area sees its fair share of them," Gensini said.

Here's what to know about hailstorms, how they form and whether those hailstones are going to get any bigger.

How does a hailstorm form?

As with any other severe weather, Gensini says a hailstorm requires four atmospheric ingredients:

• shear (the change in wind speed and/or direction over short distance)
• lift (air moving upwards)
• instability (warm air from the ground rising and meeting cold air up in the atmosphere)
• moisture (humidity in the air)

And while Lake Michigan regularly influences the Chicago region's weather, it doesn't make much of a difference when it comes to hail since the stones form far up in the atmosphere, Gensini said.

What impacts a hailstone’s size?

Most hailstones are small, pebble-sized chunks of ice, Gensini said. But it's challenging to track the average size of a hailstone because they begin to melt as soon as they touch the ground.

That vertical blast of air is especially key for determining how big a hailstone becomes, he says.

The stronger the upward air flow, the longer the hail hangs in the atmosphere before falling to the ground, Gensini says. That allows the stones to grow larger and collide with other stones. That collision is also why some hailstones are oddly shaped and others are more round.

Is climate change making hail worse?

As with other extreme weather, there isn't a simple answer, Gensini said.

"In an event like this week's storm, climate change certainly played a role," Gensini said. "But as a scientist, I'm like, well, how much of a role —2% of a role, 5% of a role, 10% of a role? Those are questions that are incredibly difficult to answer."

Gensini and his colleagues published a paper in 2024 exploring climate change's future impacts on hail.

They found that since climate change is shown to cause stronger upward blasts of air, it's likely that larger hailstones will become more normal. But because of warming temperatures, smaller hailstones will become less frequent because they'll melt before they hit the ground.

"The science currently is very immature," Gensini said. "We have a lot of work to do in terms of attributing climate change to specific tornado or hailstorm events."

Will hail become more destructive?

What's certain, Gensini said, is future storms will cause more and more damage.

Hail already causes roughly $60 billion worth of damage in the U.S. each year, Gensini said.

"Regardless of the impact of climate change, I can guarantee you that we're going to see more hailstorm disasters in the future," he said. "It has really nothing to do with climate change. It has everything to do with the fact that ... our cities are growing larger and the bullseyes on the dartboard, so to speak, are growing larger as sprawl continues."