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PMC Overview

Polar mesospheric clouds (PMCs), also known as noctilucent clouds (NLCs), form at altitudes near 83 km during summer at polar latitudes, and can be viewed with the naked eye.  When viewed from the ground, NLCs appear in reflected sunlight against the dark twilight sky. These sightings can be brilliant, leading ground-based observers to call them noctilucent or ”night-shining” clouds. NLCs consist of tiny water ice particles, and thus are controlled by temperature and humidity. Mesospheric temperature and humidity respond to changes in CO2 and CH4, gases that are increasing due to human activity. As a result, NLCs are a visible indicator of global climate change, and indeed evidence shows that NLC patterns are changing. A recent report shows that the number of NLC sightings has increased by 100% during the past 35 years [Gadsen , 1997].  Historically, NLCs have only been viewed at latitudes between 50° to 65° (in both hemispheres).  In recent years, however, NLCs have been sighted at mid-latitudes for the first time.  A recent report in the Journal of Geophysical Research [Wickwar et al., 2002] describes NLCs over Logan, Utah, that were viewed with the naked eye, photographed, and measured by lidar ("light detection and ranging"). Changes in NLC patterns have sparked an increase in the awareness of these clouds, and motivate an increased effort to understand these clouds and what they are telling us about our environment.