One rainy day in Colorado in 1998, the weather radar failed. Though the storm was mild overall—particularly in urban areas, where weather gauges were stationed—certain canyons got over nine inches of rain. Flash flooding followed, and lives were lost. 

From this event, a network of volunteer weather watchers called the Community Collaborative Rain, Hail and Snow Network, or CoCoRaHS, was born, according to local coordinator Debbie Clarkson. Their mission is to fill in microclimate data that local radar systems can’t detect. The program now includes all 50 states, as well as Puerto Rico. 

But in Marin the number of volunteers is low; compared to over 100 in Sonoma, the county has a paltry 22. The network is hoping to expand these ranks, and is offering an orientation in San Rafael on Friday.

“Marin and Sonoma Counties have all kinds of different microclimates,” Ms. Clarkson said. “I live in Rohnert Park and sometimes I’ll have half an inch or more rain than someone on the other side of town.”

Weather radar works by bouncing radio waves off of the target—in this case, raindrops, hailstones and snowflakes. The type of signal that returns to the rotating dish-like radar antenna tells operators where there is precipitation; however, as was discovered in Colorado, this system can’t pick up on small-scale weather events.

This is where the volunteer network comes in: citizen-scientists are sometimes the only ones monitoring local weather. The data they collect is useful to both confirm the accuracy of radar results and expand the information available to agencies such as the National Weather Service and the National Oceanic and Atmospheric Administration.  

In addition to reporting daily precipitation as recorded in rain gauges, volunteers also have the option of reporting anomalous weather. 

“If there is heavy or unusual weather in the area, we can turn in a report at any time of day and it will set off an alarm of sorts down in Monterey and let them know, ‘Hey, this is happening up here,’” Ms. Clarkson said. 

Of the existing Marin volunteers, only two are in West Marin: one in Dillon Beach, and one in Tomales. Yet it’s along the coast where some of the most variable weather is likely to happen. 

“Because of the mountainous coastal landscape, any weather radar in the Bay Area, including our own, might not be telling the whole story about how much rain is falling in a certain location,” said Mark Strudley of the National Weather Service. “Rain gauge data from CoCoRaHS observers helps us fill those gaps. The more gauges, the better.”


A free orientation for prospective volunteers starts at 4 p.m. on Friday, Oct. 24 at the Marin County Civic Center Board of Supervisors Chambers (Suite 330), in San Rafael. Advance registration is not required.