Hail fell vigorously in Boulder on June 12, 2023, with some ice balls as large as one inch in diameter. Credit: John Herrick

This June has been an exceptionally wet one — with rain levels already reaching roughly 3 inches, which is around 2 inches above the monthly average. Much of that fell in less than one hour on June 12, in a storm that caused significant flooding and challenged Boulder’s vulnerable stormwater infrastructure. 

It wasn’t just rain that dropped from the sky that day, however — but also hail, with some ice balls as large as one inch in diameter.

“There was just hail piled up on the gutters and on the bike lanes,” Jennifer Ho, the director of the Center for Humanities and the Arts at CU Boulder, said.

“There are all these little dents in the hood of my car, which I feel like is a rite of passage when you live in Colorado.”

Indeed, it is. 

Although it may have seemed unusual, Daniel Swain, a climate scientist at the Institute of the Environment and Sustainability UCLA, who lives in Boulder, said hailstorms in June are a rather common occurrence in Colorado. 

“Along the Front Range of Colorado in general, hailstorms, like the one [on June 12] in Boulder happen essentially every year,” Swain said. “In fact, this is exactly the time of year when you would expect to see hailstones like this.”

This is something that often takes people by surprise, he added. “You get big hailstorms in places like Texas and summer. Obviously, it’s very hot and humid in Texas. And you can get hailstorms in Florida in summer — obviously not a cool place to air either.” 

So, why does this happen? 

Swain explained that high in the Earth’s atmosphere — think 30,000 feet, a jet airliner’s cruising altitude — it’s extremely cold, below freezing temperatures. Late spring and early summer in Colorado are the times of year when big and tall thunderstorm clouds reach up to those heights. Meaning, during the June 12 thunderstorm, the “tops of the clouds were not too far from the cruising altitude of jet airliners,” Swain said. 

And when tall thunderstorm clouds form at those high altitudes, powerful updrafts of wind occur within them. These thunderstorm clouds contain “hydrometeors,” which are essentially individual particles of precipitation. Instead of falling like typical raindrops, these particles are repeatedly carried upward by the wind within the clouds. As a result, they freeze multiple times, eventually turning into ice balls.

“If these hailstones make multiple passes up and down … without falling to the surface, they can get pretty big as some of the ones did” this week, Swain said. 

While not unusual in Colorado, this year, downtown Boulder just happened to be at the center of the storm — which is unusual. 

“It’s definitely something that I don’t think downtown Boulder has seen in years,” Swain said. “It’s a little bit random year to year, in which individual towns see events like this. This year, I guess, was Boulder’s year.”

Hope Munoz is a summer 2023 Community Reporting Fellow for Boulder Reporting Lab. She is a senior at CU Boulder. Munoz can be reached at hope@boulderreportinglab.org.

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