The agro-climatic environments in subtropical islands around Japan were analyzed by using meteorological data (average for 1971-2000). This study found that the horizontal distribution of monthly mean temperatures in both the subtropical islands and Japan′s main islands can be expressed accurately as a function of latitude for all seasons. These relationships cannot be applied to the monthly mean temperatures in the eastern areas of the Asian continent except for summer. On the other hand, the annual mean diurnal ranges of temperature in the subtropical islands are around 5°C, which are about only half of those in the inland areas of Japan′s main islands. The ocean has the effect of reducing the temporal and spatial variations in temperature due to its large heat capacity. The strength of this effect depends on the temporal and spatial scale of temperature variations. That is, although the thermal effect of the ocean to the diurnal temperature variations is restricted to the subtropical islands and the coastal areas of Japan′s main islands, the seasonal temperature variations in all areas of Japan are under the influence of the thermal environment of the Pacific Ocean. The ocean also contributes to supplying water vapor into the atmosphere, resulting in more precipitation over all areas of Japan. Water balances between precipitation and potential evaporation at several sites were estimated in order to elucidate the hydrological properties in the subtropical islands.