Abstract
In a summer half year, there are two rainy seasons ; Changma (early summer rainy season) and Kaul Changma (early autumn rainy season) in Korean Peninsular, correspond-ing with Baiu and Shurin respectively in Japanese Islands and with Maiyu in early summer in China.
In this paper, the writer analysed two rainy seasons by employing the occurrence frequency of bad weather index (Lee, 1974) and synoptical bad weather day. The bad weather day was classified synoptically as follows (Fig. 2) ;
1) rainy bad weather day (_??_ sign in Fig. 2) ; the day with ≥1.0 mm of precipi-tation, ≥threshold value of daily cloud amount and ≤ threshold value of sunshine duration or the day with ≥ 1.0 mm of precipitation and ≥threshold value of daily cloud amount.
2) cloudy bad weather day or scanty rainy bad weather day (_??_ sign) ; the day with ≥ threshold value of daily cloud amount and ≤ threshold value of sunshine duration.
3) showery bad weather day (⊕ sign) ; the day with ≥ 1.0 mm of precipitation and ≥ threshold value of sunshine duration or the day with ≥ 1.0 mm of preci-pitation.
4) cloudy bad weather day (_??_ sign) ; the day with ≤ threshold value of daily cloud amount.
5) scanty sunshine bad weather day (_??_sign) ; the day of C threshold value of sun-shine duration. The determination of the threshold values was referred to in the previous paper (1974).
Though the beginning of Changma is very irregular year to year, it dates on June 24 normally. The end of Changma dates on the second decade of July in the southern region and on the last decade of July in the central region of Korean Peninsular. The phenomenon of Changma gradually moves to north as the polar front advances to higher latitude. The seasonal trend of Changma has a various pattern according to the meridional shifting of the polar front (Fig. 5 and Fig. 2). The phenomenon of seasonal intermission of summer by returning of the polar front from higher latitude is also a typical bad weather situation of summer season.
Kaul Changma is a natural seasonal phenomenon which is caused by the stagnation of cold front from higher latitude and the stationary front. But, the Maul Changma season has shorter duration than the Changma season and its seasonal trend is very irregular year to year. In some years, Kaul Changma fails to appear.
Owing to the direct influence of the continental anticyclone, there is almost no autumn rainy season in China. In Japanese Islands, the Shurin season is much longer and receives plenty of precipitation because it is also a typhoon season. Put its appear-ence pattern is intermittent according to the stagnation of southward shifting cold front from higher latitude and the stationary front, and the bad weather climatic situation is very irregular year to year.
The position of polar front is intimately related to the advance and retreat of the North Pacific anticyclone as is shown in Fig. 7 and Fig. 8.
