THE DIVISIONS AND CHARACTERISTICS OF THE NATURAL SEASONS OF JAPAN

The fourteen natural seasons are subdivided based on the occurrence frequencies of the daily pressure patterns during the period of 1941-197O as follows: _??_ Spring, summer, autumn and winter are 82 days, 91 days, 97 days and 95 days respectively. For each subdivided season, the mean values of climatic elements, such as air pressure, temperature, daily minimum temperature, daily maximum temperature, vapour pressure, relative humidity, cloud amount, wind speed, precipitation and sunshine hours are also calculated for the four stations; Sapporo, Niigata, Tokyo, Kagoshima, which are climatologically representative stations in Japan. (Early spring) Mean air temperature, the difference between maximum and minimum temperatures and sunshine hours increase from the preceeding season to this season. The combination of the pressure patterns of migratory anticyclones and troughs appears frequently. (Spring) Mean air temperatures and the differences between maximum and minimum temperatures are both larger than those in the early spring season. In the areas of Japan Sea side and the northern part of Japan, relative humidity and precipitation are decreasing. On the contrary, these values indicate an upward tendency in the southern part of Japan and Pacific side. The sunshine hours grow longer. The pressure pattern of migratory anticy-clones is prolonged in contrast to the rare appearance of this combination in early spring. (Late spring) Air pressure decreases. Sunshine hours increase. Especially in Kagoshima, it shows a peak during this season. The maximum values of wind speed during a year appears in Sapporo. The combination of pressure pattern of trough and continuous migratory anticyclone occurs most frequently. (Early summer) The maximum of sunshine hours during a year appears at this time except in Kyushu. The precipitation in Niigata shows a minimum value. The cloud amounts during this time are comparatively small. The continuous trough patterns are common. (Bai-u) The values of relative humidity (higher than 80%), cloud amount, and precipitation reach their maximum, and the sunshine minimum. The daily maximum temperatures are higher than 20°C. These conditions make this season gloomy. The continuous pressure pattern of stationary fronts is characteristic in this season. (Summer) From Bai-u to summer, sunshine hours suddenly become longer for example, it changes from 5.0 hours to 7.9 hours in Kagoshima. The mean values of temperatures increase. The lowest values of air pressure and the patterns of continuous south-high/north-low (summer type) is abundant. (Late summer) The maximum of temperature appears in this season or in summer. Vapour pressure and sunshine hours also attain the maximum. Precipitation, relative humidity and wind speed show smaller values. The combinations of pressure patterns are almost the same as those in summer. (Early autumn) Temperature, vapour pressure, relative humidity, and sunshine hours begin to decrease, but air pressure contrarily increases. In the northern part of Japan, it turns to the season of Aki-same. The combination of the continuous pressure patterns of stationary fronts which are similar to the Bai-u season is most dominant. The continuous typhoon pattern ranks second. (Aki-same) A peak of precipitation appears, except in southern Japan. Sunshine hours decrease rapidly relative to that in early autumn. It is interesting to note that the continuous pattern of migratory anticyclone predominates during this rainy season. The continuous frontal pattern also prevails. (Autumn) Air pressure increases rapidly. Temperatures decrease and the daily range becomes larger. Relative humidity and vapour pressure tend to downward. (Late autumn) Air pressure shows the highest value. Air temperature and vapour pressure decrease. Sunshine hours are shorter than that in autumn.