Noctilucent clouds, also known as Polar Mesospheric Clouds (PMCs), are a fascinating atmospheric phenomenon that occur at altitudes of 80-85 km (50-53 miles), just a few kilometers below the mesopause, which is the coldest part of the atmosphere.
These unique clouds are composed of extremely small ice crystals, measuring about 0.1 micron (1/10,000 mm) in diameter. Unlike larger ice crystals that exhibit iridescent effects, the small size of the crystals in Noctilucent clouds scatters sunlight, giving them a distinctive bluish coloration. Interestingly, the bluish hue may be attributed to the absorption of red light by the stratospheric ozone layer. On occasion, these ethereal clouds can display shades of reds and golds when illuminated by low sunlight.
The formation of Noctilucent clouds requires a combination of specific conditions, including very low temperatures, a source of water vapor, and nuclei for ice growth.
Ice does not form at the low pressures of the mesopause unless the temperature falls below -123°C. These extremely low temperatures occur only during a few weeks around the summer solstice. The occurrence of such cold temperatures during the summer is an intriguing consequence of global circulation in the middle atmosphere. Additionally, even lower temperatures (reaching approximately -160°C) have been observed a few kilometers above the cloud level, suggesting that ice crystals initially form in this region.
While the exact source of water vapor for Noctilucent cloud formation is not yet fully understood, several possibilities have been proposed. The mesosphere is known to be extremely dry, but it is believed that some water vapor may be transported across gaps in the tropopause and then lifted upward by atmospheric gravity waves. Another potential source of water vapor is methane, which reacts with hydroxyl radicals (OH) in the stratosphere to form water molecules. It is worth noting that rocket exhausts have also been associated with specific instances of later cloud formation, although this is not considered a significant factor.
Crystal Growth Nuclei
Determining the source of nuclei for crystal growth in Noctilucent clouds presents its own challenges. Extraterrestrial meteoric dust has been suggested as one possible source. Additionally, volcanic and tropospheric dust are considered potential contributors. Interestingly, the first recorded sighting of Noctilucent clouds in 1884 occurred shortly after the eruption of Krakatoa, hinting at a possible connection.
In recent years, there has been evidence suggesting that Noctilucent cloud displays are becoming more frequent, brighter, and visible at lower latitudes. It is speculated that human activities may be influencing these changes. The summer mesopause, where these clouds form, appears to be cooling, possibly due to the cooling effect of increased atmospheric carbon dioxide concentrations. Furthermore, methane concentrations and mesosphere humidity are on the rise. To further investigate Noctilucent clouds, their formation, and potential links to global climate, the AIM spacecraft was launched in 2007.
Noctilucent clouds continue to captivate scientists and sky watchers alike, offering glimpses into the dynamic nature of our atmosphere. As our understanding of these enigmatic clouds evolves, further research will undoubtedly shed light on their intricate formation mechanisms and their significance within the context of our changing climate.
Noctilucent clouds are also called Polar Mesospheric Clouds, PMCs. They are 80-85 km high (50-53 mile) a few km below the mesopause, the coldest part of the atmosphere.
NLCs are comprised of extremely small ice crystals some 0.1 micron (1/10,000 mm) diameter.
We see them by sunlight scattered by the crystals which are not large enough to show iridescent effects. Their bluish coloration is likely a result of absorption of red light by the stratospheric ozone layer. Occasionally they show reds and golds from the colour of low sunlight illuminating them.
NLC formation requires a combination of very low temperatures, a source of water vapour, and nuclei on which ice can grow.
Ice does not form at the low pressures of the mesopause unless the temperature is below -123°C. These low temperatures only occur during a few weeks around the summer solstice and the surprising combination of summer and low temperatures is a consequence of global circulation in the middle atmosphere. The very lowest temperatures (down to ~ -160°C ) occur a few km above the cloud level and it is surmised that the ice crystals initially form there.
The water vapour source is not known with certainty. The mesosphere is extremely dry but some water might be carried and across gaps in the tropopause and lofted upwards by atmospheric gravity waves. Another potential source is methane. This reacts in the stratosphere with hydroxyl radicals, OH, to form water molecules. Rocket exhausts deposit water into the mesosphere and some have been associated with specific later cloud formation but this is not considered a major effect.
Crystal Growth Nuclei
The source of nuclei is equally problematic. Extraterrestrial meteoric dust has been proposed. Volcanic and tropospheric dust is another possibility and the first recorded sighting of NLCs in 1884 was shortly after the Krakatoa eruption.
NLCs displays are thought to be becoming more frequent, brighter and visible at lower latitudes. Human activities might be contributing. The summer mesopause is getting colder, possibly because of the cooling effect of increased atmospheric carbon dioxide concentrations. Methane concentrations and mesosphere humidity are also increasing. The AIM spacecraft was launched in 2007 specifically to investigate NLCs, their formation and possible links to global climate.
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"About Noctilucent clouds - Polar Mesospheric Clouds". Atmospheric Optics. Accessed on December 10, 2023. https://atoptics.co.uk/blog/about-noctilucent-clouds-polar-mesospheric-clouds/.
"About Noctilucent clouds - Polar Mesospheric Clouds". Atmospheric Optics, https://atoptics.co.uk/blog/about-noctilucent-clouds-polar-mesospheric-clouds/. Accessed 10 December, 2023
About Noctilucent clouds - Polar Mesospheric Clouds. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/about-noctilucent-clouds-polar-mesospheric-clouds/.