This dataset documents a nine-year study to investigate the seasonal and annual dynamics of frozen ground and explore the relationship with environmental factors. The presence/absence of frozen ground and active layer thickness were recorded three times over the growing season (4-7 July, 1-4 August, and 20-24 August) at 51 permanent points in different plant communities and on 12 palsas (Thorhallsdottir 1996, p. 237). Sample size is variable: 33 permanent measurement stations in 1983, 29 in 1984, 63 in 1985-1991. Active layer depth measurement sites averaged 51 over the span of the data set. Data include mean active layer depth (and standard error) in early July and early August (for palsas and with palsas excluded); monthly degree days above zero, precipitation, snow depth, and frost degree days (i.e., below zero.) Data are presented on the CAPS Version 1.0 CD-ROM, June 1998.