Photographs have been taken at night from an airplane at an altitude of 20 km looking directly down on the tops of thunderclouds illuminated by lightning. The hard, cauliflower-like appearance of the clouds gives evidence that strong convective activity is present. In one case a well-organized system of convective structures is evident whose deepest folds, apparently caused by downdrafts, are estimated to extend into the cloud for depths of as much as several kilometers. Often the whole cloud top, approximately 10 km across, is diffusely illuminated by lightning that is occurring lower in the cloud. In most of these cases no lightning channels can be seen, but occasionally a few segments of channels are visible bridging the folds between the convective protuberances. A few photographs show thin, weak, lightning channels that come out of the top of the cloud, proceed horizontally for several hundred meters, and then terminate in the clear air above the cloud. When such channels can be seen, the background is usually quite dark, indicating that not much lightning activity is taking place elsewhere in the cloud at that time. In one unusual photograph the only evidence of luminous activity is a small region of diffuse illumination no larger than a few hundred meters in diameter. The occurrence of such small, weak discharges is evidence that small pockets of high-density space charge can accumulate within the cloud. The lightning flashes that emerge from the tops of tall, penetrative cells may introduce oxides of nitrogen and other chemical by-products directly into the stratosphere.

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* State University of New York at Albany, Albany, NY 12222

George C. Marshall Space Flight Center, National Aeronautics and Space Administration, Huntsville, AL 35812

New Mexico Institute of Mining and Technology, Socorro, NM 87801