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What are Noctilucent Clouds?
Night Shining Clouds
Noctilucent clouds (NLCs), also called night shining clouds, are tenous clouds located in the high altitudes of the mesosphere. Because of their location at such high altitudes, NLCs clouds are too faint to be perceived; however, NLCs can be observed just a little after sunset or before sunrise when they´re reached by the sunlight.
NLCs were observed for the first time in 1885 two years after the eruption of Krakatoa volcano in Indonesia. The formation of NLCs is not completely understood; however, their incidence seems to be increasing, and it is theorized that this could be in close relation to climate change.
NLCs are composed of tiny crystals of water ice whose diameter is greater than 100 nanometers, and are found at altitudes ranging between 70-85 km (43-53 miles). Unlike clouds situated in lower altitudes of the Earth´s atmosphere whose water vapor condenses around dust particles, NLCs condense directly on water vapor. It is thought that NLCs could derive from the products of volcanic eruptions, reaching the mesosphere.
Noctilucent Clouds After Sunset
Brilliant Electric Bluish Against the Black Sky
NLCs were first seen after the eruption of Krakatoa volcano, in 1885. which is considered to be among one of the most violent explosions in modern history. The explosion ejected millions of tons of ash and rock into the atmosphere, some of which may have reached the mesosphere, supplying the required nuclei for water vapor to condense. Soon after the event, NLCs were able to be seen worldwide.
T.W. Blackhouse, a skywatcher who´s credited for the discovery of NLCs declared that soon after the sun had set, there were nights when he could observe fine threads of glowing electric bluish color against the dark sky. The science community of that time concluded that it was the natural effect of the volcanic eruption. Eventually, the ash settled and the brilliant nights ended.
Ever since their first manifestation in 1885, NLCs have not only continued to persist, but have expanded, as well. One century ago, these clouds could only be observed at northern latitudes higher than 50°; however, more recently, they have been spotted in Utah and Colorado at latitudes ranging the 40°; moreover, in the present, NLCs seem even brighter and they have been spotted in the sky during the day.
Noctilucent Clouds Made up of Ice Crystals
The mesosphere, where the NLCs are found, is not only tremendously cold -125° C (257° F), but also very dry, questioning the formation of ice crystals in such an inhospitable environment. It is believed that the Krakatoa eruption may have ejected volcanic ashes, providing enough condensation nuclei for the formation of NLCs back in 1885; however, that doesn´t justify the manifestation of NLCs in present days.
One theory that could explain the source of nucleic particles in the mesosphere could be that the debris from comets and asteroids, which the Earth around its orbit is continually sweeping, is providing the nuclei necessary for the formation of noctilucent clouds.
Moisture from Lower to Higher Altitudes
Noctilucent Clouds Most Commonly Seen in the Summer Skies
NLCs are most commonly seen in the summer. This could be explained from the fact that during the summer the upwelling winds carry moisture from the lower altitudes to the higher ones, reaching the dry environment of the mesosphere. Some believe that global warming could be causing the formation of noctilucent clouds in the sky. Greenhouse gases, besides warming the Earth´s lower atmosphere, contribute to the decrease of the temperature at higher altitudes which is fundamental for the formation of ice crystals in the mesosphere.
For the study of this phenomenon, NASA launched in 2006 the Aeronomy of Ice in the Mesosphere known as (AIM). AIM will study the formation of NLCs to gain a better understanding at its relation to climate change. With this aim in mind, AIM will shoot wide angle photos and will monitor the chemical and thermal properties of the environment within clouds.