Hailstorms in the Kanto-Koshin District (3)

Farmers in some regions of the Northern Kanto believe existence of so called hyo-michi (hail street or road of hail), along which hailstorms pass frequently. They say a hyo-mich locates along a river, or a valley or a high-voltage power line. Neither reasons for formation nor the existence itself of hyo-michi have been scientifically investigated. Determination of such preferred routes of hail clouds is very useful in making plans for field observations and modification experiments of hailstorms. In this study attempts are made: (1) to determine locations of hyo-michis, (2) to study reasons for formation of such preferred routes of hailstorms, and (3) to examine year to year variation of frequency distributions of hail occurrence. The results are as follows: (1) It is found out that most of hyo-michis in the Kanto-Koshin plains originated from relatively isolated mountains or mountain groups. But in mountaneous regions, they are located along long axes of elongated basins or sometimes originated major mountain passes (Fig. 5). Hyo-michis coincide with preferred routes of thunderstorms. It is found out that only in a small areas parailelizm between hyo-michi and high voltage power lines exists. Generally speaking there is no special relation between them. (2) It is pointed out that movements of hailstorms is strongly influenced by orography in mountaneous regions. The directions of movements of hailstorms developed 21-23 June 1968 (Fig. 6) are compared with the vertically averaged wind (V: 900-300mb) field for the same period (Fig. 7). It is seen that storm movements were poorly related to the general V pattern. In mountain areas each storm moved more or less parallel to the preferred routes of the region it developed. (3) The 10 year period chosen for this study is devided into three periods: I) 1960-62; II) 1963-66; III) 1967-69 (Fig. 8). It is pointed out that a distinct hail maximum of northern Nagano Prefercture in the 10 year frequency chart (Fig. 1) is attributed to hailstorms of the period III only. In Ueda-Saku basin hail occurred frequently during the period I and II, but in the period II practically no hailstorm occurred in this area. Only region where hail occurrence was relatively uniform throughout the 10 year periods is southern Gunma northern Saitama region, where several hyo-michis with different orientations come togather.