PREVAILING WINDS IN THE ISHIKARI PLAIN, HOKKAIDO

Abstract

The W-NW winds from the Japan Sea prevail in the Ishikari Plain in winter and the S-SE winds from the Pacific Ocean in summer. In order to make clear the detailed distribu-tion of such prevailing winds, the writers observed the wind-shaped trees at the 180 points as indicators of wind speeds and directions. The wind-shaped trees observed were those of Fraxinus mandshurica, Alnus japonica, Larix leptolepis, Populus spp., Picea jezoensis etc. and the grade of deformation was determined as illustrated in Fig. 1. The results obtained are summarized as follows : (1) In summer (Figs. 2 and 3): (a) The southerly winds from the Pacific Ocean go up to north in the central part of the Plain along the Yubari mountains and then gradually change the direction to northeast wards between the Mashike mountains and the eastern moun- tain regions. (b) One branch of the southerly winds flows northwestwards from the central part of the Plain to the Japan Sea. Another branch flows into the low land area around Yuni between the Yubari mountains and the Umaoi hilly region. (c) The strongest wind region was found in the area arround the City of Ebetsu, the central part of the Plain. The other, small, strong wind regions were distributed in several parts, , as caused by the local topogra phical conditions. (2) In winter (Figs. 4 and 5): (a) The northwesterly winds, the winter monsoon, prevail over the Plain. Their southern border was located at Oyachi, but the eastern border runs fur-ther inner side along the foot of the Yubari mountains. (b) The westerly winds, after crossing over the Mashike mountains, change their direction slightly and blow northeastwards between the Mashike and the Yubari mountains. (c) It was confirmed, from the distribution of remain- ing snow on May 1_??_6, 1970, that the so-called local “Ishikari-front” runs roughly along the line connected from Tobetsu to Iwamizawa, as shown in Fig. 6. (3) The results mentioned above were compared with the results, given in Figs. 7 and 9 for summer and in Figs. 3 and 10 for winter, which were observed instrumentally at the 20 Agrometeorological Stations distributed in the area studied. A striking good coincidence was seen for the conditions of southerly winds in summer. Of course, the distribution map obtained by observing the wind-shaped trees revealed the detail, because of the number of observation points. For the conditions in winter, however, it is considered that the prevailing wind conditions estimated by the wind-shaped trees of ever green needle trees such as Picea jezoensis do show the distribution of the winds with snow. Therefore, it can be said that the winds carrying snow are the prevailing winds in winter in a broad sence, but they are not always the most frequent winds in some places.