Monsoon Precipitation Area and Cyclonic Precipitation Area in Japanese Winter Season

The winter monsoon brings a lot of snow on the Japan Sea side of Japanese Islands. On the contrary, the Pacific side of Japanese Islands has very fine weather. Therefore, weather boundaries between the Japan Sea side area and Pacific side area are important as the division line of climatic provinces. But, because the position of the weather boundaries differ from day by day, there are many different representations of climatic division. The writer defined the climatic division line with a new method. The ratio of amount of monsoon precipitation to monthly amount of precipitation in January, and the ratio of number of days with monsoon precipitation to number of days with precipitation in January were chosen as the measures of climatic division. As the result, the Japanese Islands were divided to three climatic porvinces, that is, the monsoon precipitation area, the cyclonic precipitation area and the transitional area