This paper gives climatological information on hail occurrence in Kanto-Koshin District. Hailstorms occur mostly in the warm season in this area, therefore, climatology of warm season hailstorms is studied in detail.
Data of the recent eitght years are used to obtain a frequency distribution, for it is possible to determine hail patterns for most of hailstorms occurred during this period. As shown in Fig. 4, there are three distinct maxima and two secondary maxima in the frequency distribution. It is noteworthy that all of these maxima are situated about 70km east through southeast of major mountain areas.
Locations of hailswaths observed during the eight year-period are examined. The result (Fig. 5a-5d) indicates that the local farmer's tradition “hailstorms always take the similar routes” is not entirely untrue. A number of interseting features may be pointed out from the result, some of them are as follows:
1) Most hailstorms in this region move east through southeastward. However, in the western region most hailstorms are eastward moving, whereas eastern region there are more southeastward moving storms.
2) Hailstorms moving east-southeastward are associated with thunderstorms developed near the NW-Gunma Mtns or the Nikko Mtns, whereas those moving northeastward are associated with thunderstorms developed near the Chichibu Mtns.
3) Hailstorms move from mountain areas toward lower lands, they rarely cross big mountain regions, and sometimes change courses to follow valleys.
Finally, long term variations of hail occurrence are studied. Although long term increase of hail days seems to exist (Fig. 6), at least partly, it is attributed to the tendency that fewer records remain for earlier days. However, the records for recent 20 years indicate increase in hailstorm activities druing the last several years. Computations of correlation coefficients between different areas and stations are made. The result suggested that for long term view point, hail occurrence is quite random.
