When heavy rain and a flash flood devastated the Colorado State University campus and Fort Collins community on July 28, 1997, it would’ve been hard to imagine that a long-lasting positive outcome would emerge. Yet that is the history of the Community Collaborative Rain, Hail, and Snow network, or CoCoRaHS, which is celebrating its 25^th anniversary this week. June 17 has officially been proclaimed as “CoCoRaHS Day” in Colorado by Governor Jared Polis.

After the 1997 rainstorm, former state climatologist Nolan Doesken set out to determine how much rain had fallen in Fort Collins. Using water collected in buckets or whatever else may have offered a reasonable estimate, he and colleagues discovered that the rain was extremely localized, with over 12” on the west side of town, but only 1” on the east side. The official rain gauge on campus recorded 6.17” of rain over two days. But Nolan realized that if local residents had relatively low-cost rain gauges outside their homes, and if there were a way to systematically collect the daily rainfall totals from these “citizen scientists”, that it would be possible to paint much more detailed pictures of future rainstorms.

CoCoRaHS began as a local effort in Fort Collins, with the first observations entered on June 17, 1998. The network expanded to the rest of Colorado, then nationwide, then internationally. CoCoRaHS now has over 26,000 volunteer observers across the United States (including all 50 states, Puerto Rico, Guam, and the US Virgin Islands), Canada, and the Bahamas, and it is the single largest source of daily precipitation measurements in the US. In 2022, over 5 million observations were collected, and there are no signs of slowing down.

CoCoRaHS volunteers are dedicated—because the official gauge holds about 11 inches of rain, stories have been shared from observers in tropical areas diligently dumping out their gauge once or twice per day during major storms to ensure accurate measurements. The data collected by CoCoRaHS observers are trusted for a wide array of applications: they are vital to National Weather Service operations in monitoring droughts and floods, by meteorologists studying rainfall and hailstorms, and as “ground truth” for researchers when calibrating advanced radar systems. Farmers and ranchers contribute measurements to the network, and also rely on the data to inform their operations.

The CoCoRaHS network is managed by a small, dedicated group of staff at the Colorado Climate Center at CSU, but the “Community” part of CoCoRaHS is truly vital to its success. In addition to sharing and comparing their rainfall measurements, CoCoRaHS volunteers connect with one another online for “WxTalk Webinars”, and even at in-person gatherings. Volunteer coordinators recruit new observers and ensure the data meet the highest quality standards. CoCoRaHS has also advanced a wide range of educational activities, encouraging schools to participate and contributing to climate and water literacy activities across the country.

And even after 25 years, the network continues to grow. If you, or someone you know, are always watching the weather, then you are a perfect candidate to become a CoCoRaHS observer. The only requirements are an official rain gauge, enthusiasm to carefully measure precipitation each day, and the excitement to be a citizen scientist.