Comments on "effect of electric charge on collisions between cloud droplets"

Fletcher's simple model for the electric force between charged conducting spheres (with reference to cloud droplets) is compared with an exact model and also with another approximate model. His results for the charge ratio as a function of the distance at which the force of attraction changes to one of repulsion differs from that of the other treatments by a factor of 4 at large separations (where the effects of approximations should be negligible) and by a factor of 8 at small separations. The reasons for the discrepancy are explored, and the other treatments are found to be reliable. The attractive force is present for a much larger range of separations, and for more meteorologically plausible charge ratios, than in Fletcher's results. The general results are discussed in the context of confounding effects of droplet inertia, which is the dominant factor in collisions unless there are very high droplet charges. Inertia is much less important for collisions of charged aerosol particles and cloud droplets, and an important application of models of electric-force calculations is in electrically enhanced scavenging in clouds that are weakly charged by current flow in the global atmospheric electric circuit.