Four Years ago, one of the present authors described the climatic divisons of Japan in this journal
(1). In determining these divisions, temperature and precipitation were considered independantly of each other. In the course of his studies he observed a marked difference in climate between Kyusyu and the Ryukyu Islands.
Last Spring, the same writer travelled through the last-named islands in order to make detailed observation with these ploblems, the results of which may be summarized as follows:
At Yaku Sima (130° 30'E. 30°20'N.), which lies south of Kyusyu beyond Van Diemen's Strait, a mild climate persists throughout the year. The laterite like soil that lies upon the marine terrace is an indication of the high temperature and great humidity that prevail in the island. The very conspicuous vertical zoning of the climate is clearly recognized in this island; and while in the highest zone subalpine characters predQminate, moderate conditions prevail thrughout the coastal lowlands.
In the Satu-nan Islands the annual march of temperature is very gentle; the maximum being in August. This August-maximum is not restricted to these islands, but is experienced in all the Japanese islands, big and small, although July-maximum is what generally obtains in almost all the other temperate regions of the world. Hence, we cannot very well regard this peculiar condition as being primarily due to the oceanicity of the climate. According to the writer's opinion, it is due to the fact that the July-temperature is lowered by the great amount ob rainfall during the “Bai-u” (a rainy season that lasts front early June until about mid-July). Dr. Okada has also pointed out the remarkable influence exercised by the Bai-u in preventing the rise of July temperature
(2).
The seasonal variation in the amount of rainfall in these islands is largely influenced by this rainy season and by typhoons, which account for the July and Sepienber maximum, respectively. The annual total rainfall ranges from 2000 to 3000mm, which is no heavier than the rainfall of Southern Kyusyu, Sikoku and the Kii Penninsula, This can be explained by the low position of the observatories in these islands. At the Kosugitani lumbering camp, located on the northeastern slope of Yaku Sima (altitude 750m), the annual precipitation reaches 8080. 8mm (1929-1930). Such a heavy rainfall has rarely been observed in other parts of Japan.
The climatic boundary between Kyusyu and the Ryukyu Islands is considered to lie along the Collet Strait, Van Diemen's Strait is also an important boundary, although of secondary significance.
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