Journal of the Japanese Society of Snow and Ice

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The occurrence of permafrost was reported on the summit of Mt. Fuji in the early 1970s. In view of recent climate change, public attention has been given to possible changes in surface and underground conditions, including permafrost, on Mt. Fuji. Over two years beginning in 2008, we monitored two ground temperature profiles down to 3m on the summit of Mt. Fuji, and we have not confirmed the occurrence of permafrost. At one site, we observed seasonal frost reaching more than 3m deep. However, heavy rain events every summer triggered large increases in ground temperature, and heavy rains during the autumn rainy season rapidly thawed the frozen layer deeper than 2-3m. At the other site, the insulating effect of snow cover weakened frost penetration into the ground, and heating by rain infiltration kept ground temperature relatively high throughout the monitored period. Our investigation suggests that the ground temperature regime varies greatly over years, and the thermal status of frozen ground is unstable on the summit of Mt. Fuji. It is difficult to evaluate the influence of recent climate change on the underground condition on the summit of Mt. Fuji using information available at this moment. Evaluation of long-term changes in the ground temperature regime and the status of frozen ground should be based on multipoint and long-term monitoring of the ground to a deeper extent, together with surface micrometeorological observations.

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Although a weak layer in deposited snow is one of the causes of slab avalanches, only a few attempts have been made to study their metamorphism over a long period. We observed weak layers of depth hoar, graupel, and rime-free crystals in Sapporo, Japan, during two winter seasons and measured the time variations of their SFI (shear frame index). The shear strength of the weak layers of depth hoar and graupel remained low in contrast to that of rime-free crystals. The thickness of the depth hoar layer did not change for an entire month. We hypothesized that the low compressibility of depth hoar is the reason for its low shear strength.

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