Soil temperature has great significances not only for the study of heat budget near the ground, but also for the practical requirement such as agriculture and soil engineering. Although many studies on soil temperature have been carried out until now, we have many problems that remain for further study. In particular, it has hardly been made clear how the soil moisture affects the heat budget near the ground. The relationship between soil moisture and thermal properties has been studied by physicists in laboratories, but it has never been completed in the practical field. Moreover, even statistical investigations have not been carried out sufficiently, especially the phenomena of a longer time scale. Therefore, in order to solve these problems, the author made observations and investigations, climatological and statistical.<br> § 1. Effect of rainfall on the diurnal variation of soil temperature<br> In the summer of 1964, the author and his collaborators tried some observations of soil temperature together with other meteorological elements at Hoya, a suburbs of Tokyo and the Kanna River bank, Saitama Prefecture. As for soil temperature, observation was conducted by thermistor thermometers for every two hours and recorded by the electric recorder (Picture). A few examples of them are as follows : Generally, except in the special case of sand soil at the river bank (Fig. 5), the diurnal change is not recognizable at less than 50 cm depth, independent on the weather (Figs. 2, 3 & 4). But in the upper layer soil temperature, it is controlled by the weather ondition, especially by rainfall. In order to make it concrete, the author calculated thermal diffusivity K² at the depth of 10 cm using following formula for every two hours:<br>_??_<br>The results are shown at the bottom of each figure. From these figures, the following facts are concluded :<br> 1) In general, the thermal diffusivity K² is large in daytime and small at night.<br> 2) On a fine day, K² is comparatively small, large on fine weather after rain and becomes smaller again on a day with continuous rain.<br> § 2. Effect of rainfall on the interdiurnal change of soil temperature<br> Next, the relationship between mean daily soil temperature and daily rainfall was investigated in the same way by using the data obtainted at the meteorological observatories. For comparison, the data at two stations in Tokyo were statistically treated. As for soil temperature, the difference from the previous day was used for Kichijoji and the deviation from the decade mean value for the Central Meteorological Observatory (Figs. 6, 7). Although the thermal diffusivity K² was calculated in the same way as in § 1, some values had negative signs probably because of a longer time scale. So the author adopted only the positive values in each graph (Figs. 6, 7). However, these positive K² were found to have an interesting relation with the daily rainfall as shown in Fig. 8. In other words, the thermal diffusivity increases or decreases according to the amount of rainfall having nearly the same tendency as in § 1.<br> § 3. Effect of precipitation on the secular change of soil temperature<br> The relation of the mean annual soil temperature at the depth of 1 m and 3 m to the annual total precipitation was also studied using the climatological data at two places. Here the author selected two sets of place which are comparatively close to each other, with different characters from the viewpoint of natural geographical conditions: Hikone-Kyôtô and Maebashi-Tôkyô (Figs. 9, 10, 12).