Experiments out of doors and in a wind tunnel show that when trees, vegetation, and other substances are burned in the open air, negative charge is release into the atmosphere under the influence of the fine weather electric field. The results indicate that the thunderclouds formed as the result of a forest fire have probably grown from air that contains negative, instead of the usual positive, fine weather space charge. Observations showing that cumulonimbus clouds formed over forest fires produced positive, instead of the usual negative, cloud-to-ground lightning, suggest that the electrification of these clouds may have been caused by one or more influence mechanisms. ¿ American Geophysical Union 1995