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CSU state climatologist's office to keep official hailstone records for Colorado

May 10, 2012

As large as a grapefruit? A DVD? A softball? Just how big is the record-holding hailstone in Colorado?

CSU State Climatologist Nolan Doesken says that there have been at least 18 separate reports of hail stones as large as 4.5 inches in diameter, but few of these reports have been officially confirmed.

Record-keeping just in time for springA storm near Idalia, Colo., in early May deposited nearly 8 inches of hail (photo by Denise Marostica).

His office is now officially tracking these statistics – just in time for spring storms like the one that hit Colorado’s eastern Plains earlier this week, leaving piles of hail on roads and farms.

“Chances are there are at least 50 people in eastern Colorado who will say, ‘What the heck, we’ve seen way bigger hail than that!’” Doesken said. “That’s good, but they will need to prove their claim – by providing the stone itself or eyewitness reports along with excellent photos showing the stone with rulers or other known objects.”

Doesken’s Colorado Climate Center at Colorado State, working in tandem with the four National Weather Service offices that serve Colorado (Boulder, Pueblo, Grand Junction and Goodland, Kan.), have developed procedures for documenting hailstones including how to get a good photo, measure the maximum diameter and circumference, preserve the stone and contact  appropriate officials. For instructions, examples and contact information, go to

In the meantime, Doesken encourages volunteers who observe large hail in Colorado to first report to the National Weather Service or to local law enforcement agencies. The National Weather Service can convey reports of damaging hail to a wider audience through National Weather Service alerts. Large-hail reports provide valuable ground truth which will be helpful for issuance of additional warnings.

“When it’s safe to go out again, retrieve the largest stones and freeze them as soon as possible to avoid further melting,” Doesken said. “Use a ruler to get an accurate measure of the diameter and a string or a flexible tape measure to determine the circumference. If an accurate scale is available, weigh the stone. Preserve the stone by tightly wrapping and sealing it in an air-tight bag.”

If the stone is 4.5 inches in diameter or greater, contact the Colorado Climate Center at (970) 491-8545 or Nolan Doesken.

Proper documentation includes preserving stone

People who have kept hailstones in their freezer will be disappointed to know the stones lose mass over time, Doesken said. Even if they are tightly wrapped, the ice structure changes with time. But changes will occur more slowly if they are sealed up air tight.

Henry Reges, CoCoRaHS national coordinator at CSU, helped NOAA transport this record hailstone from South Dakota in 2010.Doesken got the idea for a hailstone repository while he was a member of the executive committee of American Association of State Climatologists. Several neighboring states including Nebraska, South Dakota and Kansas have established new records in recent years. A stone that was eight inches in diameter fell in Vivian, S.D., on July 23, 2010 – almost the size of a football.

“I don’t believe Colorado has ever had hail that big – and I hope we never do,” Doesken said. “But stones don’t need to be that big to cause damage. You can really get pummeled with stones that are 1 inch in diameter. Even golf-ball sized hail, which most people don’t realize is nearly 2 inches in diameter, can do serious damage. Crops and gardens can be badly damaged by hail that may only be pea-sized.”

“The motivation has been that other states have been establishing these repositories and Colorado should have done this long ago,” he said. “If you’re going to get hit with something that big, you might as well take some credit for it.”

Doesken has been tracking hail data for more than 30 years. Fort Collins was one of the locations hit with 4.5-inch diameter hail on July 30, 1979.

In 1998, Doesken established a volunteer precipitation monitoring network in Colorado known as the Community Collaborative Rain, Hail and Snow Network, or CoCoRaHS. The network is now in all 50 states with more than 15,000 volunteers. Data from the volunteers improves precipitation monitoring and helps provide detailed storm analysis, drought, water supply and other water decision-making information to municipalities, homeowners, industries, utility providers, resource managers and educators. The program continues to seek volunteers. To volunteer, go to

Guidance for documenting record hail stones will be available soon on the Colorado Climate Center website at

Guide for hailstone size (diameter measurement listed):

  • Ping pong ball – 1.5 inches
  • Golf ball –1.75 inches
  • Tennis ball - 2.5 inches
  • Baseball - 2.75 inches
  • Hockey puck - 3 inches
  • Softball - 4 inches
  • Grapefruit - 4.5 inches
  • CD/DVD - 4.75 inches

Contact: Emily Wilmsen
Phone: (970) 491-2336