The continuous observations of the rain current (i), the rate of rainfall (R′) and the potential gradient (P) were made. Providing the Wilson's theory of ion capture by water drops, the quantitative representation between them is obtained using the size-distribution of raindrops by Marshall and Palmer as
i=2.12×10^-6P_hR′^0.84-0.54×10^-6P_hR′^1.05,
where i gives the maximum rain current and P_h is the potential gradient in the charging region of raindrops below the clouds. Applying the calculated result to the measured ones, it is found that P_h relates to P by |P_h|=3P^1.3 The relation between the above results and the Simpson's empirical ones is discussed. Further, some results in regard to the splashing effect of raindrops at the ground and to the mirror-image effect between the rain current and the potential gradient are described.