This dataset includes frost tube data from 37 stations in the Upper Midwest (Minnesota, North Dakota, Wisconsin), USA. The responsible agency was the St. Paul District of the U.S. Army Corps of Engineers. These data were collected during 1971-1981 (no data for 1976/77) by cooperative observers who gathered the data for use in their spring run-off hydrologic predictions. The observers had frost tubes installed by District personnel in their back yards. The early penetration of frost at the beginning of the freezing season was not observed, but most observers picked up the record when 1 or 2' of frost had occurred. This data base, a preliminary version, was constructed by Richard K. Haugen and Glenn King from the manuscript records of the cooperative observers and is presented on the CAPS Version 1.0 CD-ROM, June 1998.