A STRONG LOCAL WIND ALONG RIVER HOHKI, TOCHIGI PREFECTURE

Abstract

Along River Hohki, down the north-west or northwest on the western margin of the Nasu Fan, Tochigi Prefecture, blows a strong local wind very frequently. Especially, it is strengthened at -the area near the railway bridge of Tohoku-line crossing the river. To make clear the special conditions of this local wind, an observation was made from a microclimatological viewpoint in December, 1957, and February, 1958, after the analysis of synoptic situation. In this study, the following conclusions were obtained. (1) The strong winds, exceeding often 30 m/s (meter per second), appeared at the time when the winter monsoon was developing or the cyclone was passing along the Japan Sea coast. (2) The frequency table of the strong winds, having 10 m/s or more at its maximum speed, shows that they blew approximately at the rate of once for two days in the winter season. The average of their maximum speed reaches 13-19 m/s and their duration is frequently 1-2 hours. The extreme value was 16-20 hours. (3) As a result of the microclimatic field observation of the winds, it is clarified that when the prevailing wind direction coincided with the direction of the river, they had strong wind speeds in the area around the bridge. (4) It is because of the micro-topographic situation of the area; that is, at the southernn side of the river, there is a low hill having a cliff about 20 meters in height, and at the northern side, there exist river terraces of several meters height. (5) A great fluctuation of the wind direction and also the upward component was found at the stations near the cliff, 20-30 meters apart, as the wind velocity was relatively weak. (6) In the farther part, 50-60 meters away from the cliff, the strong winds were observed under the prevailing northwest or north-northwest wind condition, and the ratio of wind speed at the station on the bridge, 7 meters in relative height, to that at the river floor was 1.4-2.0. (7) When the prevailing wind blew at the right angle to the direction of the river course, the influence of the hill upon the wind speed distribution appeared as far as 250 meters' distance on the windward side. This effect is the same to that of a windbreak in its order, that is, about eight times of the relative height, about 30 meters, of the hill. (8) The ratio of wind speed between the two stations generally varied with the prevailing wind speed. There was found a negative relation between the ratio and the prevailing wind speeed.