The phenomenon of the slope wind caused by cool air near the ground surface of a slope at clear nights furnishes an interesting problem in micro-meteorology, and at the same time is one of the principal subjects in agro-meteorology, as it apt to generate local differences of low temperature and to cause frost damage on crops especially in cold season. Many investigations on slope wind have been carried out, but it seems that most of them have been founded on only surface observations. Hence the author tried to make observations not only in the surface layer but also in the layer up to the height of about 250 m, and to deduce some conclusions about the slope wind and allied phenomena. The observations were carried out chiefly at a small island (Hakatajima Island in the Inland Sea of Seto), whose area is about 22 km^2 and maximum height is about 304 m from the mean sea level, and partly at a large slope of a mountain (Ogi-yama, Beppu City, Oita Pref.) in order to obtain some comparative data. Though these observations were made from 1949 to 1964, only the results of 1961-1963 are given as the objects of investigation in this paper. The paper is made up of two parts, the first part deals chiefly with the phenomenon of the slope wind and may be said to be micro-meteorological, and the second part concerns with the temperature distribution associated with the slope wind and the frost damage on crops and hence has a character of agro-meteorology. The results of t h e observations are summarized as follows: /A. Concerning the slope wind itself/ (1) The down slope wind on clear and calm nights arises immediately after sunset and lasts till the sun rises next morning, so long as there is no remarkable change in weather condition. The wind velocity, in many cases, is one meter per second and the wind goes on blowing at an almost constant speed all night long. (2) The depth of the down wind layer in a small valley is 25-30 meters thick, and the air temperature there falls till about 4℃, the maximum wind velocity being generally found in the five meters height. (3) The down slope wind found on ridges of either side of a valley and also in the upper part of the valley itself is weaker in strength and more irregular than that in the valley, but has the characteristics of down slope wind blowing down along the slope, the thickness being small and not constant. (4) The observed down slope wind coincides with Prandtl's theory either in regard to the vertical distribution or to the magnitude of its maximum value, but the observed air temperature distribution does not agree with the theory. The reason for the discrepancy is not easy to explain the fact that, though the theory treats the two-dimensional phenomenon, it is three-dimensional in the actual case, seems at present to be the only possibility of explanation. /B. Concerning the influence of the general wind upon the d own slope wind/ (1) When the general wind rises in the inverse direction, the inversion layer grows weak and the velocity of the down slope wind falls. The relation between velocities of the winds is generally expressed by a negative linear function, and when the general wind attains to the velocity of about 1.5 m/s, the down slope wind disappears. (2) Similar relation holds also when the general wind rises in the same direction with the down slope wind, i.e., the latter grows weaker as the former becomes more remarkable. The wind velocity in the valley reaches a minimum and after then increases, showing that the general wind predominates also in the valley. /C. Concerning the circulation of the down slope wind/ Synthesizing the results of the wind in and above a valley of the north-west peninsula at Hakatajima, a return current against the down slope wind was evidently found to exist. As the wind velocity was very weak (less than 1.0 meters per second) and easily disappeared when the general wind intervenes, the domain of the return current was difficult to decide exactly, however, the height of its lower and upper limits seemed to exist between 50-110 m and 100-180 m respectively above the coastal region.
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