stereographic cloud analysis from Apollo 6 photographs over a cold front

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Abstract
Twenty-three overlapping photographs from the Apollo 6 mission were stereographically analyzed to determine the three dimensional cloud structure over a strong cold front in the southeastern United States on 4 April 1968. The analysis extended from near Tyler, Texas, eastward to the Georgia coast. Four major cloud regions were observed: a zone of altocumulus behind the front, a vigorous thunderstorm complex along and just ahead of the front, a chaotic region of multilayer clouds for a few hundred kilometers east of the thunderstorms and a large region of cloud streets parallel to the surface winds near the coast. The cloud top altitudes in the thunderstorm complex were extraordinary, reaching a maximum height of 22.375 km, whereas the tropopause was located at 12–13 km and the maximum tops reported by radar 30 min before the Apollo pass were at 12.5 km. An area of approximately 2 × 104 km2 was covered with maximum tops of ⩾ 18 km. The accuracy of the stereographic technique was confirmed with cloud top altitudes computed from cloud shadows either on lower clouds and/or on the ground with the Apollo photographs or with ATS-3 images that were taken earlier when the sun angle was lower and the shadows longer. The agreement was within 500 m for the Apollo comparisons which is about the expected error for each method. When the ATS pictures were used the shadow error was undoubtedly larger due to the lower sensor resolution and lower contrast between the shadow and ground but the 18–20 km cloud top heights agree with the altitudes calculated by the stereographic technique for the western edge of the thunderstorms almost 2 h later. A total of 2.0 × 109 kg of water vapor was estimated to be injected into the lower stratosphere if the air was 50% saturated with respect to ice within the cloud volume and if 1% of the available ice was vaporized within the stratosphere from clouds that had a 1 g/m3 density that half filled the stratosphere below the calculated cloud top levels.

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