Mesospheric cloud observations at unusually low latitudes |
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| Institution: | 1. Sleep Disorders Centre, Department of Respiratory & Sleep Medicine, Princess Alexandra Hospital, Ipswich Rd, Woolloongabba, Queensland, Australia;2. Faculty of Science and Engineering, Queensland University of Technology, Queensland, Australia;3. Department of Clinical Neurophysiology, Seinäjoki Central Hospital, Seinäjoki, Finland;4. Department of Applied Physics, University of Eastern Finland, Kuopio, Finland;5. Diagnostic Imaging Center, Kuopio University Hospital, Kuopio, Finland;1. Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences, Fudan University, Shanghai 200438, China;2. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing 100081, China |
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| Abstract: | Noctilucent clouds (NLC) are a beautiful, high-latitude, summertime phenomenon that was first reported over 100 years ago. They are seen during the hours of twilight by the scattering of sunlight from sub-micron-sized ice particles that form in the vicinity of the cold mesopause region. NLC are quite distinctive, often appearing silvery-blue in color. In recent years there has been a marked increase in their frequency of occurrence, possibly due to an increase in mesospheric water vapor and/or to a cooling of the mesopause region, prompting speculation that they are “harbingers” of potentially serious changes in the mesospheric climate. In concert with this trend there are also a growing number of ground-based NLC sightings at significantly lower latitudes than expected. Here we report two unusual NLC displays photographed from Logan, UT, USA (～42°N) during June 1999, well over 10° lower in latitude than expected and implying a major, yet temporary, departure from normal mid-latitude summertime conditions. These data provide new evidence for the occasional expansion of NLC to unusually low latitudes possibly due to exceptional dynamical forcing. Alternatively, they may be an early indicator of significant long-term changes taking place in the upper mesospheric summertime environment. |
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