The seagull-type crystal is a non-six-fold rotational symmetry type of snow crystal. Although the seagull-type crystals are commonly observed in the arctic areas of the cryosphere, we observed them at Fukiage onsen in Kami-Furano Town in March 2007. They were also observed at Wakkanai City in December 2020, which can be considered a rare event except for in the mountainous regions of Japan. At the time of observation, the temperature was －9.6 °Con the ground and －27.7 °Cat the top of the cloud (2106 m). The observed crystals may have formed in the lower troposphere due to the strong cold air over Wakkanai City.

Permafrost, which is widely distributed in cold regions, has been greatly affected by recent climate change. Thawing of permafrost, especially in mountainous areas, can change the stability of the ground. Clarifying the dynamics of mountain permafrost is important for constructing climate change adaptation measures, and so is the development of monitoring methods for these measures. In this study, field observation and Interferometric Synthetic Aperture Radar (InSAR) analyses were conducted to detect seasonal thaw subsidence in the Daisetsu Mountains, Hokkaido, considered to have the widest mountainous permafrost in Japan. A field observation carried out during the snow-free season in 2020 resulted in more than 2 cm seasonal subsidence from the end of May through June, followed by more than 0.5 cm in subsidence through the end of August. InSAR results have shown that spatial variation in subsidence coincided with that of wind-swept terrains with almost no vegetation and have the same tendency for change as the field observation data, confirming the effectiveness of InSAR. However, the magnitude of seasonal thaw subsidence in May—June by InSAR was less than 1 cm, smaller than the field observation. This difference is considered result from the difference in spatial representativeness between the leveling survey and InSAR.

Microplastics (MPs) have been found not only atmospheric deposition in urban areas, but also the snow obtained from Mt. Everest and Arctic, sediments in the deepest part of the Mariana Trench.

However, the pathway of the MPs to the atmosphere has not been clear. Here, we collected rime-ice and snowpack samples at Mt. Karakunidake in winter season to clarify the existence of atmospheric MPs deposition　in rime-ice and snowpack, using FTIR imaging. MPs were detected in the range of 8.34×10⁶ m^－3 to 12.3×10⁶ m^－3 in rime-ice, and 1.34×10⁶ m^－3 in snowpack. The concentration of MPs in rime-ice was about 10 times higher than that in snowpack. The particle size distribution of MPs in rime-ice showed more than 90.1 % of MPs were present below 100 µm fraction, and most of them were fragments. The predominant plastic found in the samples is polyethylene (as fragments). Mt. Karakunidake is isolated from the impact of human activities. This indicates that MPs can be transported from other area.

The paper reviews experimental and theoretical studies on regelation. Cohesion of contacting two pieces of melting ice was found in 1850 by Faraday. Although its mechanism remained undefined, a new experiment using a weighted wire through ice was conducted (Bottomley, 1872). This penetration phenomenon was explained by pressure melting and refreezing. Experiments using ice slabs were then conducted over years, until a new method using a semi-cylinder of ice was introduced in 1973 (Drake and Shreve). Quantitative analysis has advanced the understanding of the relationship between the wire speed and the driving stress, the wire diameter, and the thermal conductivity of the wire. The speed was in proportion to pressure at pressures greater than 0.13 MPa. However, analysis revealed that the speed was extremely low in a region under pressure of 0.13 MPa. On the other hand, the speed was derived theoretically under the assumption of uniform water thickness around the wire, and pressure melting and refreezing (Ornstein, 1903; Nye, 1967). (i) For nylon wires, heat conducts mainly from ice, and the estimated speed was twice the observed speed. (ii) For low thermal conductive wires such as chromel wires, the estimated and observed results were in good agreement. For both (i) and (ii), the speeds were roughly independent of the thickness of the water layer. (iii) For high conductive wires such as copper wires, the speed was remarkably higher than the observed speed. The temperature balance was not achieved because the water layer was too thin, and the viscosity η was too small. If the value η was 125 times greater than that of the bulk water, the layer would become five times thicker, and the calculated speed would agree with the observed speed. Water vapor was often observed on the upper side of the wire, indicating that atmospheric pressure never affects the wire. The load is supported only on the underside, and then pressure melting will occur from a tripple point, 0.01 °C．A transition region for speed can be interpreted by no or partial melting. Future tasks include the measurements of the distribution temperature and pressure around wire and the viscosity of the thin water film.

The freezing conditions of Lake Mashu from 1974 to 2021 were investigated at the nearest observatory. We found that Lake Mashu was completely covered by ice 27 times during the period. The complete ice coverage rate was 56.3 %. We also found that Lake Mashu was completely covered by ice at 95.5 % when the average monthly air temperature in February at the nearest meteorological station (Kawayu AMeDAS) decreased below －8.9 °C. When the yearly minimum air temperature of simple moving averages using 61 average daily temperatures at the second nearest meteorological station (Teshikaga AMeDAS) decreased below －7.8 °C, we also detected the complete coverage of Lake Mashu. The accumulated negative average daily air temperature for the complete ice coverage condition is not constant and depends on the air temperature of the previous summer. Using the relationship, we can forecast the freeze-up date of Lake Mashu in 2021. The forecast date was February 24, 2021 based on the forecast on September 1, 2020. We can also forecast the dates as February 10 and February 8, 2021 based on the forecasts on January 1 and January 16, 2021, respectively. Lake Mashu was completely covered by ice on February 14 in 2021, with the forecast errors of ＋8, －4, and －6 days for the respective forecasts. Thus, the prediction is achieved within about±1 week. Because the average monthly air temperature in February increases in this region, the complete ice coverage percentage of Lake Mashu will decrease in the future.