This is weather peculiar to the eastern regions of latitudes 28°-33°N., roughly comprising the Middle-Lower Changjiang River (Yangtze River) valley, the southern Huaihe valley, Zhejiang and Fujian provinces. They come very year between late spring and early summer for about a month at a time when plums ripen. They are also called “ mould rains” because things easily go moldy in the long wet spell. Although the timing varies from year to year and from place to place, the season generally begins in mid-June and ends in mid-July. As it coincides with rice-transplanting time, the rainfall is favourable to the growth of the rice although excess may result in water-logging.

Around this time, the southeast monsoon has gathered force and driven the warm, moist air current to the Yangtze River and Huaihe river valleys but it is not strong enough to blow it farther north. On the other hand, although the cold air current from the north has weakened in force, it has not withdrawn from these valleys. When the two different currents meet in the Yangtze River and Huaihe river valleys, neither is strong enough to drive the other out. This causes the rain belt to linger and consequently gives rise to intermittent drizzles until early or mid-July, when the warm current prevails over the cold current and drives it back to north China.