In this paper the author investigates the correlation of the difference of sunshine duration between the Jordan and the Campbell sunshine recorder, denoted by s, with the transmission coefficient of the atmosphere, and a large negative correlation is found. Thus, s expresses the degree of turbidity of the atmosphere.
Next the positive correlation of s with the summer temperature in Tohoku districts is found. The summer temperature in Tohoku districts is, generally, high or low according to whether they are under the prevalance of the Ogasawara air mass (tropical maritime air mass) or the Okhotsk air mass (polar maritime air mass). It is evident that the turbidity is larger in the former than in the latter. Therefore the above statistical result is naturally reasonable.
Lastly the multiple correlation of the summer temperature in Tohoku districts with s and the number of sun spots in the previous year is discussed and an experimental equation for predicting the summer temperature in the next year is derived.