日本における１７８０年代暖候期の天候推移と自然季節区分

Abstract

A study has been made to classify the natural seasons in Japan for the summer half years (May-October) in the period 1781-90. The weather diagrams and the daily weather distribution maps for that period are described using the data of daily weather records in the official diaries (Fig. 1). The duration of five seasons-spring, the Baiu (summer rainy season), midsummer, the Shurin (autumn rainy season) and autumn-for each year is determined based on the seasonal march of weather (Fig. 2).Average beginning dates of the seasons for 1781-90 were not so different as those for today. However, the year-to-year variability of the beginning dates and the length of natural seasons for that period was large. Among those years, 1783, 1785 and 1786 had extremely unusual weather situations.The climate in 1783 was characterized by the remarkably cool and damp weather conditions in summer (Fig. 3). The beginning and ending dates of the Baiu season were approaximately as usual, whereas the following midsummer, which began on 19 July and ended 4 August in northeastern and central Japan, was extraordinarily short. During the Shurin season which started as early as on 5 August and ended on 10 September, the cloudy and rainy weather caused by the cyclonic and frontal activities prevailed, especially in the northeastern and central Japan. The great historic famine known as “Tenmei no Kikin” could be attributed to those severe weather situations.By contrast, the summer of 1785 was exceptionally hot and dry (Fig. 4). The Baiu season began on 3 June and ended as early as on the beginning of July, and this was followed by the extremely long midsummer which lasted until 3 September. A long spell of hot and dry weather in midsummer was occasionally interrupted by the short spell of stormy weather brought by the approach of typhoons. Those unusual weather conditions caused severe droughts in central and southwestern Japan. The Shurin season followed by midsummer began on 4 September and a rainy weather persisted until the end of October.The weather situations in 1786 were characterized by the long spell of wet weather in the Baiu season and the unstable weather in midsummer (Fig. 5). The Baiu season, the end of which was delayed until the beginning of August, was remarkably long and wet, particularly in southwestern Japan. The duration of midsummer was about 40 days and this was somewhat shorter than as usual. During this season, the heavy rains and floods occured frequently under the influence of typhoons. Those bad weather situations caused the poor harvest of rice cultivation.From these results, we can conclude that the climate in Japan for 1781-90 was extremely variable as for the summer weather situations.