Abstract 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 ba...