The role of ionic diffusion in placing free electrical charge on droplets falling in the atmosphere is considered. Experimentally verified expressions for the diffusion of ions onto droplets are shown to account for the sign and magnitude of the free charge observed on light rain and mist. In the lower atmosphere, such droplets are normally positive and carry a specific free charge approximating 1 esu/gm. The observed variability of the electrical conductivity of the atmosphere suggests that diffusion may occasionally transfer large negative as well as positive charges to the falling drops.