T he Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) originated in the aft ermath of a fl ash-fl ood storm that dropped more than 12-in. of rain over a small portion of Fort Collins, Colorado, on 28 July 1997, and a similar storm the following evening over the grasslands of northeastern Colorado. Th ese fl oods were responsible for several fatalities and at least $200 million in property damage. Neither event would have been accurately recorded by existing networks of offi cial weather stations. National Weather Service (NWS) radar also failed to assess the severity of these storms (Kelsch 1998a,b; Petersen et al. 1999). However, when the citizens of these areas were asked to help, they enthusiastically provided scientists with a wealth of information from their own backyard observations. Th ese data eventually resulted in the very accurate mapping of precipitation from the Fort Collins fl ash fl ood that continues to be used by engineers, hydrolo-gists, weather forecasters, city planners, emergency managers, attorneys, teachers, historians, and many others for such applications as flood forecasting, drought monitoring, verifi cation of radar-estimated precipitation, and climate trends. Encouraged by the enthusiastic public response, versity (CSU) in Fort Collins began mobilizing a network of citizen rain and hail observers in the spring of 1998. Th e initial plans targeted teachers and students from all grades in Fort Collins. However, unexpected free publicity from local media spread the word to the general public. Within a matter of days over 150 people volunteered, including students from nearly 40 schools in the region and interested adults. What started as a collaborative data-collection venture has developed into an exciting community based science education program that includes classroom presentations, fi eld trips, training sessions, picnics, and informal seminars. CoCoRaHS now includes over 1000 active volunteers across Colorado (see Table 1), as well as several hundred in Nebraska, Wyoming, Kansas, and New Mexico. They range in age from 6 to 80-plus years. Th is diversity of age and background of participants is one of the true TABLE 1. Level of participation in the CoCoRaHS network (Colorado only) from 1998 through Oct 2004. Active volunteers are defined as observers who submit at least five observations between 1 Jun and 31 Oct for the calendar year. In addition to the numbers below, several hundred more observers participated for a few weeks but have not remained active.