formation are an extremely cold mesopause (Theon et al., 1967) and sufficient ambient water vapor and numerous condensation nuclei (Witt, 1969). If these conditions are met anywhere in the upper atmosphere, it is at the high latitude summer mesopause where they would be expected to be most likely. And it is here that NLCs were observed by satellite (Donahue et al., 1972). By contrast, NLCs are not seen at latitudes below 45° except for artificial NLCs correlated with SCOUT missile launches from Pt. Mugu (Meinel et al., 1963) and with French sounding rockets (Benech and Dessens, 1974). A detailed discussion of NLC sightings and morphology can be found in Fogle and Haurwitz (1966). By depositing water vapor and exhaust particles near the mesopause and thereby enhancing two of the three conditions believed necessary for NLC formation, rockets can apparently form artificial mesospheric clouds (Meinel et al, 1963; Benech and Dessens, 19/4) and might presumably increase the frequency or extent of natural NLCs. Rocket injections of water vapor may also lower the temperature at or near the mesopause. Chenurnoy and Charina (1977) studied variations in hydroxyl (OH) band emissions before, during, and after an NLC display. As a result they suggested that higher concentrations of ^0 before formation lead to enhanced OH emission causing local cooling; after formation of the NLC, H2O vapor is condensed into cloud particles leading to reduced emission. Despite the observed production of artificial mesospheric clouds following rocket launches, as cited above, two direct mesospheric releases each of 2 kg H2O over Ft. Greeley, Alaska, on 5 and 8 August 1964, failed to produce observable cloud (Fogle et al., 1965). However, since the water was released in bulk at heights that could not be clearly determined, these failures may not be significant. Liquid water would not have much time to evaporate and condense into particles of sufficient size to make a visible cloud. (See also the discussion of Section 2.5.3 concerning several additional high altitude water release experiments, and the discussion in item F.2 of Appendix F on the specific enthalpy of different water releases). Novozhilov (1979) has suggested that the formation of NLCs is probably facilitated by the presence of a deep cyclonic vortex in the mesosphere, and Scott (1974) reports that NLC sighting occur earlier in summers following major sudden stratospheric warmings. 2.4.2 Nacreous Clouds (Ellsaesser) Nacreous clouds form in the stratosphere (~ 25 to 30 km) in the region of temperatures below —80°C that develop within the polar winter vortices, particularly over Antarctica. As such they are indicative of (high) stratospheric water vapor mixing ratios and (low) temperatures, and of year-to-year variations in atmospheric circulation. In view of the dearth of information of this type, they offer an additional source of information for interpreting or inferring conditions at high levels in the atmosphere. The catalog of sighting by Stanford and Davis (1974) shows periods of maximum frequency in the 1880s to 1890s and in the 1930s to 1940s. There was also a 30-year period from 1895 to 1926 during which almost no sighting were done. These data suggest long-period cycles of variation in stratospheric
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