The hourly precipitation observations contained in this set are obtained as months become available from the National Climatic Data Center (NCDC). The reports are taken by observers at principle (primary) stations, secondary stations, and cooperative observer stations operated by the National Weather Service (NWS) and the Federal Aviation Agency (FAA). The sites are located in the contiguous U.S., Alaska, Puerto Rico, Virgin Islands, Hawaii and other Pacific Islands.

While most stations begin observations in 1948, some have observations as far back as 1900. There are currently more than 2500 stations reporting. Periods of record vary widely among the stations.