“…These extreme cases are not alone in the NLC statistics; for example, high-latitude out-ofseason NLC observations were reported by Lohvinenko and Zalcik (1991) on 10 September 1988, from Cape Parry, Canada (70:2 3 N) and by Warren et al (1997) from the South Pole on 29 April 1992, as late as four months after summer solstice. The exact geophysical causes for these unexpected NLC formations had not been identified and still remain unclear, but these might be attributed either to the formation of unusually cold pockets of air, for instance, due to gravity wave breaking, or to large amounts of water vapor release from a Space Shuttle or large rocket launch (Stevens et al, 2005).…”