Largest volunteer weather observation network

The largest volunteer weather observation network is CoCoRaHS (the Community Collaborative Rain, Hail and Snow Network), which collected data from 26,500 precipitation measuring stations (a total of 5.6 million daily reports ) around the United States, Canada and the Bahamas in 2023.

CoCoRaHS has its origins in a flash flood that swept through Larimer County, Colorado, on 27 July 1997. The flood on the Spring Creek swept through a mobile home park in Fort Collins, killing five people. Meteorologist Nolan Doesken, a lecturer at nearby Colorado State University, was shocked by the fact that no weather stations had registered the massive rainfall that had triggered the flood, and that consequently no warnings had been issued.

The problem was that the rainstorm that fed the flood was extremely localized, dropping 36 cm of rain in an area only a few kilometres wide. Doesken began asking his students and local volunteers to set up precipitation gauges in areas that weren't covered by professionally maintained weather stations, with the first stations being established in 1998. Initially, the aim of this project was to get higher-resolution coverage in the state of Colorado, but it began to expand to other states in 2003, and has since grown to include stations in all 50 states as well as Canada and the Bahamas.

CoCoRaHS volunteers are provided with a standard plastic rain gauge (essentially a funnel mounted over a measuring tube) and educational materials on how to site and monitor the gauge.