The freezing and thawing of the active layer of soil in the Arctic terrestrial drainage basin is simulated using a one-dimensional heat conduction model. Main forcing data are surface air temperature and snow thickness. Emphasis is placed on producing a topography-improved temperature data set from National Centers for Environmental Prediction (NCEP) reanalysis data, and a snow height data set compiled from SSM/I satellite data and observed climatological snow density. Soil bulk density, soil type and its relative composition are derived from the SoilData System of the IGBP-DIS. The model is run with 25 km ¿ 25 km resolution and daily time steps for the period September 1998 through December 2000. We compare modeled thaw depths and active layer depths with those measured at Circumpolar Active Layer Monitoring (CALM) field sites. This study shows for the first time the highly variable daily thaw depth over permafrost and seasonally frozen ground depth in nonpermafrost areas, as well as freezing and thawing periods for the whole pan-Arctic landmass.