  Cloud droplet mobility is referred to here as a measure of the droplets
ability to move with ambient air. We claim that an important part of the
aerosol effect on convective clouds is driven by changes in droplet mobility.
We show that the mass-weighted average droplet terminal velocity, defined here
as the effective terminal velocity (eta) and its spread (sigma_eta) serve as
direct measures of this effect. Moreover, we develop analytical estimations for
eta and sigma_eta to show that changes in the relative dispersion of eta
(epsilon_eta = sigma_eta/eta) can serve as a sensitive predictor of the onset
of droplet-collection processes.
