CoCoRaHS Hail Observations


To collect hail observations, CoCoRaHS participants use the simplest of devices: a 1-in thick styrofoam pad covered with heavy duty foil to measure the number and size of hailstones.

Observers can quickly submit a hail report that includes starting time, approximate hailstone size (max, min, average), the texture of the hailstone (i.e., hard vs. soft), and any evidence of damage. Later, volunteers can edit their report to add more detail to describe the duration of the event, size and depth of the hailstones, occurrence of rain with the hail, approximate fall angle of the hailstones, and evidence of damage.

The hail and intense rain reports are sent directly from the CoCoRaHS server to the Local Data Acquisition and Dissemination server at the NWS office in Boulder. If criteria such as a certain hail size or rainfall rate are met, the report is reformatted and sent to the appropriate NWS offices. NWS workstations can be set to alarm on receipt of this report, ensuring that the warning forecaster sees the data immediately. These reports are used for nowcasting and for the issuance and verification of flood and severe thunderstorm warnings. As CoCoRaHS continues to expand, the data will be sent to other NWS offices in real-time.

After a hailstorm, damaged hail pads are collected from volunteers. The pads, not the hail reports, are the final data source inventoried at the CoCoRaHS office. Paid student interns examine each hail pad and measure and count the size and number of dents and relate dent size to actual hailstone size. These data are entered into the archive along with digital images of each hail pad. All data, including individual daily entry reports, summaries, maps (rain, snow, snow water content, hail occurrence and size), and the digital hail pad images, are available at the CoCoRaHS Web site.

In addition to daily maps, a variety of data reports are produced that allow quick and easy analysis of frequency, magnitudes, and areal extent of rain, snow, and hail. Because data quality is a paramount concern, we've taken a number of steps to ensure data accuracy and consistency.