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Mesospheric Cloud Observationsat Unusually Low Latitudes

Document Type

Article

Journal/Book Title/Conference

Journal of Atmospheric and Solar-Terrestrial Physics

Volume

64

Issue

8-11

Publisher

Elsevier

Publication Date

5-2002

First Page

991

Last Page

999

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.

Comments

Published by Elsevier in Journal of Atmospheric and Solar-Terrestrial Physics.

http://www.sciencedirect.com/science/article/pii/S1364682602000536

Publisher PDF is available for download through the link above.

https://doi.org/10.1016/S1364-6826(02)00053-6

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