Vigorous debate still surrounds the cloud electrification process and unexplained regional variations in lightning activity. Here, we show that climatological maxima in lightning activity are associated with small effective diameter De of ice crystals near cumulonimbus cloud tops. This relationship, unlike lightning's more well-known relationship with cloud top height, is consistent over land and ocean. Since multiple studies indicate that De is reduced by atmospheric aerosol, this relationship strengthens previous suggestions of a role for aerosols as well as dynamics in electrification. Moreover, the angular distribution of backscattered radiance shows that modest (~10%) De decreases reflect large (~2¿) increases in the number of small (<~30 ¿m) particles N, a finding supported by cloud model simulations. Both relationships provide an important new test of cloud microphysics and/or electrification models.