There are three major reasons why CoCoRaHS devotes so much time and effort in monitoring rainfall:
Precipitation (especially rain) is highly variable in space and time;
Existing observing networks are too far apart to see much of this variability; and
Rainfall reports can make a big difference in our communities: they help determine the health of food crops, provide an indication of soil and drought conditions, and they can literally save lives during periods of heavy rainfall. Because CoCoRaHS intense rain reports are sent directly to the National Weather Service (NWS), our observations also help NWS forecasters track storms and have, on more than one occasion, provided guidance for issuing flash flooding warnings.
How is Rain Measured?
To measure rainfall and snow water equivelent, particpants use a standard 4-inch diameter rain gauge. The rain gauges are manufactured by Productive Alternatives in Minnesota. Preliminary comparisons at the Colorado State University (CSU) campus weather station in Fort Collins, CO suggest that the 4" CoCoRaHS gauge has a collection efficiency of 101-105% in rain with respect to the NWS 8" standard rain gauge (SRG). The difference appears to be that the SRG (copper) absorbs some precipitation before it enters the inner measurement cylinder. In snow, it is likely that the CoCoRaHS gauge has reduced collection efficiency relative to the SRG, especially in open areas.
Other Things of Interest Regarding Rainfall
To find out how rain gauge data is used by meteorologists to validate numerical forecast models of precipitation, click here
To find out more about rainfall, flooding, and surface runoff, click here
To see a 1971-2000 average precipitation map for the U.S., click here
To find out how weather radar is used to measure precipitation, including rainfall, click here.
Because data quality is a paramount concern, we've taken a number of steps to ensure data accuracy and consistency.