Freeze–thaw cycles on rock surfaces below the timberline in a montane zone: Field measurements in Kobugahara, Northern Ashio Mountains, Central Japan

Rock-surface temperatures were measured on a tor and a large block located below the timberline in the Kobugahara Highlands, an upland plateau composed of granodiorite in central Japan (36°N, E139°, 1300 m a.s.l.). Many blocks in the region originate from core stones distributed on the plateau, and there are several block streams in the valley bottoms. The tor is situated at the upper end of one of these block streams. The large block appears to have detached from another tor. The mean air temperature of the coldest month is − 4 °C. Rock-surface temperatures were recorded continually every 30 min from December 1, 2002 through December 13, 2004 by thermistor probes connected to miniature data loggers. The diurnal and the annual freeze–thaw cycles and effective freeze–thaw cycles were recorded at each site. The freezing index of rock-surface temperature showed a marked variability from year to year. In addition, differences in local conditions at observation sites within a given area greatly affected the rock-surface temperature. These are important considerations in relation to local weathering below the timberline.

Research highlights
► The temperatures of the air and rock surface were measured on slopes below the timberline in a montane zone of central Japan (36° N, E139°, and 1300 m a.s.l.).
► It is clear that annual changes in the freezing index are large, and that there are large variations in the numbers of FTC and EFTC within the same area.
► These differences are primarily caused by differences in the depth of snow cover.