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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.
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.
Motion pictures have been taken at night by astronauts on the space shuttle showing lightning discharges that spread horizontally at speeds of 105 m·s−1 for distances over 60 km. Tape recordings have been made of the accompanying optical pulses detected with a photocell optical system. The observations show that lightning is often a mesoscale phenomenon that can convey large amounts of electric charge to earth from an extensive cloud system via a cloud-to-ground discharge.
Motion pictures have been taken at night by astronauts on the space shuttle showing lightning discharges that spread horizontally at speeds of 105 m·s−1 for distances over 60 km. Tape recordings have been made of the accompanying optical pulses detected with a photocell optical system. The observations show that lightning is often a mesoscale phenomenon that can convey large amounts of electric charge to earth from an extensive cloud system via a cloud-to-ground discharge.
In order to determine how to achieve orders of magnitude improvement in spatial and temporal resolution and in sensitivity of satellite lightning sensors, better quantitative measurements of the characteristics of the optical emissions from lightning as observed from above tops of thunderclouds are required. A number of sensors have been developed and integrated into an instrument package and flown aboard a NASA U-2 aircraft. The objectives have been to acquire optical lightning data needed for designing the lightning mapper sensor, and to study lightning physics and the correlation of lightning activity with storm characteristics. The instrumentation and observations of the program are reviewed and their significance for future research is discussed.
In order to determine how to achieve orders of magnitude improvement in spatial and temporal resolution and in sensitivity of satellite lightning sensors, better quantitative measurements of the characteristics of the optical emissions from lightning as observed from above tops of thunderclouds are required. A number of sensors have been developed and integrated into an instrument package and flown aboard a NASA U-2 aircraft. The objectives have been to acquire optical lightning data needed for designing the lightning mapper sensor, and to study lightning physics and the correlation of lightning activity with storm characteristics. The instrumentation and observations of the program are reviewed and their significance for future research is discussed.
