Gentle mass movement caused by the freeze-thaw process is very important on slopes in the alpine zone. In Japan, the mass movement process and weathering caused by the freeze-thaw process are being clarified by field experimental research. However, on the actual conditions of air temperature and soil temperature which provide for freeze-thaw processes, little research has been done, and the results are not reported in detail. Because of this the author observed air temperature and soil temperatures at 2 cm, 5 cm, and 10 cm depths continuously on the top slope of Mt. Hoo, in the northeastern part of Japan's Southern Alps (Figs. 1 and 2). The observation period was from September 24, 1986, to June 8, 1987 (258 days; absences: 15 days).<br> As a result of this observation, the changing pattern of air temperature 1.5m above the ground surface is seen to be similar to the temperature of soil at 2cm depth. The trend of 5cm depth soil temperature is similar to that of 10 cm depth soil temperature. The diurnal cyclic freeze-thaw process is evident in air temperature and 2 cm depth soil temperature (Fig. 4). This is caused by surface rubble layer distributed below 2 cm depth (Fig. 3). The air temperature affects the 2cm depth soil temperature. However, the 2cm depth soil temperature rises over air temperature because of solar radiation, and it actually promotes the thawing process in the 2cm depth soil layer, even when the air temperature is below 0°C. This process notably occurs in the thawing period.<br> The 5cm and 10cm depth soil temperatures show cyclic changes which are not directly affected by air temperature. During the observation period, 5cm and 10cm soil temperature stabilized at -1.5_??_-2°c for a specified period. The stabilization is caused by latent heat release when soil freezes and latent heat absorption when soil thaws; this is the so-called zero-curtain phenomena.<br> Changes in soil temperatures are steeper in thawing periods than in freezing periods (Fig. 5). Soil temper-atures rose sharply and dropped gently during the observation period.<br> In the thawing period, the range of diurnal change in the 2cm depth soil temperature exceeds that in air temperature caused by solar radiation. As a result of this process, the number of freeze-thaw days at 5cm and 10cm depths increases in the thawing period. The 2cm depth soil temperature rises before the air temper-ature rises in the latter half of the freeze period and the first half of the thaw period. This occurs due to solar radiation. The small-scale fluctuation of air temperature and 2cm depth soil temperature above 0°C is caused by temporary masking of solar radiation by clouds or fog.<br> As described above, on the top slope of Mt. Hoo, the solar radiation has a very strongly effects on the mass movement of the dopes. This tendency occurs most markedly near the surface.