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[1] Polar Mesospheric Clouds (PMCs) were observed by a limb-scanning ultraviolet spectrometer on the Student Nitric Oxide Explorer (SNOE). Radiance profiles at 215 and 237 nm are analyzed to determine the presence of clouds. Once detected, the altitude and brightness of a cloud relative to the background atmosphere is determined. SNOE observations provide the frequency of occurrence of PMC as a function of location and time for the years 1998 through 2003. The observations show at high latitudes a general rise in frequency of occurrence beginning approximately 3 weeks before summer solstice in both hemispheres and lasting for approximately 1 week. These rises are followed by approximately 60 days of relatively high but variable occurrence frequencies. The declines in frequency of occurrence at the ends of the seasons are generally slower and more structured then the beginning of the seasons. One of the major results from the SNOE observations is that significantly more PMCs are observed in the Northern Hemisphere than in the south, leading us to conclude that the southern polar mesosphere must be on average less saturated than the northern polar mesosphere. The SNOE observations also suggest that the frequency of occurrence of PMCs is strongly modulated by local dynamical influences. The SNOE results are in general agreement with results from the Solar Mesosphere Explorer which observed PMC with similar instrumentation in the years 1981 through 1986.

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