The average transition of the daily minimum water-surface temperature of Lake Nakanoumi and that of the daily mean air temperature of its shore area are recognized to be about the same expect in winter.
Winter is the rainy season in this district, and a large quantity of relatively cold rainwater flows into this lake, so that the water-surface temperature seems to keep a low level in comparison with that of the air temperature. This phenomenon makes the warming effect of this lake in winter poor.