The results in Fig. 3 establish that our cloud model can reproduce many of the observed general features of NLCs [see Turco et al. (5) for more detailed comparisons], Moreover, it does so without recourse to unreasonable physical assumptions; e.g., unusually high water vapor abundances or exaggerated upward convective velocities in the mesosphere. Instead, ambient sporadic clouds are seen to be a natural manifestation of high-altitude meteorology (e.g., 4). The model outlined here, therefore, should be suitable for a preliminary assessment of the changes in mesospheric cloudiness induced locally by individual rocket launches and worldwide by water vapor buildup over many years of rocket activity. 3. OBSERVATIONS OF ARTIFICIAL HIGH-ALTITUDE CLOUDS Meinel et al. (11) reported brilliant mesospheric clouds to the west of Tucson, Arizona on seven evenings during the first 6 months of 1963. The sightings were unusual in that natural NLCs are never observed at such low latitudes. Meinel et al. correlated the incidents with rocket launches in southern California, and attributed the clouds to water vapor and particulates deposited near the mesopause by rocket engines. They likened the clouds to NLCs. Similarly, in February, 1963, McDonald
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