Three Years of Rahs – Uh, Cheers – for Collection of Climate Data

After three years of volunteer effort, Colorado State University has amassed a collection of data that constitutes a good start toward understanding Colorado’s unpredictable weather.

Thanks to volunteers from the Colorado Collaborative Rain and Hail Study, or CoCoRaHS, detailed information on rain and hail storms now exists for Larimer County. Moreover, the project, supported by the Colorado Climate Center at Colorado State, is making inroads in the city of Greeley and Weld County, Boulder County and several eastern Colorado counties.

Assistant state climatologist Nolan Doesken, director of CoCoRaHS, said the project is a practical science education experiment in which Colorado residents can participate. Initially staffed by students recruited from Fort Collins schools, the effort now has an interesting mix of participants from young people to seniors.

The volunteers measure rainfall amounts, hailstone sizes and intensity and report daily via the Internet to the Colorado Climate Center. So, after gathering three year’s worth of precipitation data on Fort Collins, what have the volunteers found?

"I have to say the results have been so simple they sound silly – they sound like we should be saying ‘duh’," Doesken said. "We knew some of what we were going to find, but from a scientific, climatological basis, we’d just never been able to show it so clearly. Precipitation is just amazingly variable. Recognizing that variability, even looking for patterns in the chaos, helps us understand storm dimensions and behavior.

"Based on that understanding, we can begin to see things like how much water the cities need for lawn watering, where those blasted mosquitoes will hatch and bite and, because of inclement weather, which ball games will be canceled and which won’t."

In addition, CoCoRaHS data helps the National Weather Service predict and verify storms and serves as backup checks for experimental radar observations being conducted by the CHILL Doppler radar operated by Colorado State’s Department of Atmospheric Science and Department of Electrical and Computer Engineering.

Ironically, CoCoRaHS was being planned and tested in July 1997 when a deadly flood struck Fort Collins, killing five city residents.

"It really took the Fort Collins flood to get people interested in what the project could do and to get financial support for CoCoRaHS," said Doesken, who later calculated that rainfall in that storm varied from 3 inches to 14.5 inches within a three-mile distance.

After starting with a few dozen volunteers in 1998, CoCoRaHS has grown to more than 360 active participants in 2000 with participants from Larimer, Weld, Adams, Morgan, Boulder, Kit Carson, Yuma, Cheyenne and Washington counties. Similar local efforts also have emerged in Colorado Springs and Denver.

Doesken envisions a statewide network of volunteer reporters who can assess rain, hail and snow for the whole state. In addition to its scientific value, the data could help Colorado’s agricultural and recreational industries.

CoCoRaHS has determined, for example, that variability exists not just over the state but from year to year. About 18.5 inches of rain fell in Larimer County between April and September 1999, according to project volunteers, while only 9.3 inches fell during the same period in 2000.

Each volunteer needs a professional rain gauge, donated if the project has received sponsor support or sold for about $25, and a supply of hail pads (over 1,000 were manufactured this year and donated by a local Boy Scout troop). Dents from hailstones can be measured and counted on the hail pads, which are 12-inch-square, three-quarters-inch thick Styrofoam pads covered with aluminum foil.

Doesken says the most widespread hail along the Front Range typically occurs in June, but since the project began, April and May have produced the most storms. However, early storms cause little damage because most hailstones are from one-eighth to three-eights inches in diameter, many stones are soft and crops are just beginning to grow.

Conversely, later storms can pack more punch. Doesken has a pad with its foil cover ripped and Styrofoam deeply dented, struck by stones up to 2 inches in diameter. The pad was given to him by a volunteer nine miles northwest of Greeley after a storm that hit Sept. 1, 2000.

"There are theories that some localized areas are more prone to hail than others, but until we actually measure localized long-term weather behavior, the theory remains just that," he said.

Larimer and Weld county participants are now being trained to measure snow.

"We tend to think that snowfall is more uniform that rain, but we are quickly finding that snowfall patterns are also surprisingly localized," Doesken said.

"If we can just get this project going in a long-term sense, we’ll be able to get a better understanding of Colorado’s precipitation on a microscale basis," he said. "We’ll be seeing more improvements in radar data, too."

To join in the effort to document Colorado’s wild weather, apply online at (click on "Join Us") or call the Colorado Climate Center at Colorado State University at (970) 491-8545.