Enhancement of cloud-to-ground lightning over Houston, Texas

Cloud-to-ground lightning flash data have been analyzed for the twelve-year period 1989-2000, for a geographical area centered on Houston, Texas. Of t he 1.6 million cloud-to-ground flashes in this area of study, approximately 752,000 flashes occurred in the summer months of June, July, and August, a nd 119,000 flashes in the months of December, January, and February. The hi ghest flash densities, greater than 4 nashes km(-2) in the summer and 0.7 f lashes/km(-2) in the winter, are near the urban areas of Houston. We sugges t that the elevated flash densities could result from several factors, incl uding, 1) the convergence due to the urban heat island effect, and 2) the i ncreasing levels of air pollution from anthropogenic sources producing nume rous small droplets and thereby suppressing mean droplet size. The latter e ffect would enable more cloud water to reach the mixed phase region where i t is involved in the formation of precipitation and the separation of elect ric charge, leading to an enhancement of lightning.