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http://www.denverpost.com/ci_5579483...Noctilucent clouds appear only at night, when their altitude - 50 miles up in the atmosphere - let's them catch sunlight no longer visible from Earth's surface, said James Russell, an atmospheric scientist at Hampton University in Virginia, the NASA mission's principal investigator.
That makes noctilucent - "night-shining" - clouds appear almost iridescent, he said. Most normal clouds are less than 10 miles up.
"They're very beautiful, they look very mysterious, but aside from all that, these clouds are changing in ways we don't understand," Russell said.
Researchers have many ideas about why noctilucent clouds may be growing in number and brightness, and most of them are related to global warming. [continued]
...These changes have sparked a widespread interest in PMCs from many people all over the world, however some have argued [von Zahn, 2003] that the evidence for these changes is tenuous at best. If these observations ultimately prove to be accurate and persistent over longer time periods, these increases in PMC occurrence and brightness could signify long-term changes in the Earth’s climate. Since PMC formation seems to be related to water and temperature, much of the focus has been on searching for long-term changes in these parameters. Thomas et. al. [1989] reasoned that long-term increases in mesospheric water vapor are occurring based on observed increases in tropospheric methane which has led to more water for PMC formation and, hence, larger and brighter PMCs. Tropospheric methane is transported into the stratosphere where it is
destroyed in a set of chemical reactions whose output is water vapor [Le Texier et. al., 1988]. In another theory, Thomas [1996] hypothesized that ground-level CO2 increases over the last hundred years have caused a net cooling in the mesosphere creating the right conditions for PMCs to form. The indirect effect of mesospheric cooling in response to increased CO2 at the ground was illustrated in modeling studies by Roble and Dickinson [1989]. Thomas described the PMC temporal trend as the “miner’s canary” of global change meaning that noctilucent clouds could be thought of as evidence that the earth’s atmosphere is undergoing important changes.
However, Lübken [2001] has shown from falling sphere in situ measurements that the thermal structure of the high latitude summer mesosphere has undergone little change since the 1960s. Stevens et. al. [2005] have suggested that the influence of water released by Space Shuttle flights may alter PMCs to the extent that observed long-term trends (e.g. Fig. 1.2) may not be caused solely by climate change. These authors showed evidence of water released by the Space Shuttle flight STS-107 that advected to Antarctica and PMC observations observed by a lidar over Rothera, Antarctica and by the SBUV NOAA-16 and 17 satellites. Also, the ice mass contained in the PMCs over 12 days after the launch is compatible with the water amount released in the shuttle exhaust plume, thus, illustrating a connection between Space Shuttle exhaust and PMC formation. Given the large number of Space Shuttle flights in the last few decades, the effect of these flights on the water vapor budget must be considered when evaluating long-term trends in PMCs. Other open questions are PMC particle size characteristics, how dynamical and extraterrestrial forcings influence the upper mesosphere and how these processes translate into PMC formation, and PMC nucleation mechanisms.
Noctilucent clouds are thin, wispy clouds that are found in the upper atmosphere at altitudes of around 76-85 kilometers. They are made up of ice crystals and are only visible during the twilight hours.
Scientists have observed an increase in the frequency and brightness of noctilucent clouds in recent years. This is likely due to the rise in greenhouse gases, which leads to a cooling of the upper atmosphere and creates conditions that are favorable for the formation of these clouds.
Noctilucent clouds are not harmful to our environment. In fact, they play an important role in studying the upper atmosphere and can provide valuable information about climate change and the health of our planet.
Noctilucent clouds can only be seen from latitudes between 50 and 70 degrees in the Northern and Southern hemispheres. This means that they are not visible from all parts of the world and can only be seen in certain regions.
Noctilucent clouds are most easily observed during the summer months, in the hours after sunset or before sunrise. They are best seen in areas with minimal light pollution and a clear view of the horizon. Using binoculars or a telescope can also enhance the viewing experience.