PRECL Monthly Mean Rainfall and Number of Gauges in 2.5x2.5 boxes (Version 1.0).
References
Chen, M., P. Xie, J. E. Janowiak, and P. A. Arkin, 2002: Global Land Precipitation: A 50-yr Monthly Analysis Based on Gauge Observations, J. of Hydrometeorology, 3, 249-266