Noctilucent clouds (NLCs), also known as night shining clouds, are tenuous high altitude clouds. NLCs are the highest clouds in the Earth´s atmosphere, and are usually too faint to be seen. NLCs can be seen illuminated by the sun´s rays just after sunset or before sunrise. They were first observed in 1885 two years after the eruption of Krakatoa. Their formation is not well understood; however, their occurrence seems to be increasing, and it is believed that this could be related to climate change. Understanding what noctilucent clouds are can help us better understand this phenomenon.
Noctilucen clouds contain within tiny crystals of water ice with a diameter of up to 100 nanometers, and remain at altitudes in between 75-85 km (46-53 miles). Unlike clouds in lower altitudes of the Earth´s atmosphere whose water vapor collects around dust particles, noctilucent clouds form directly on water vapor. It is believed that these clouds could originate after the product of volcanic activity reaches such altitudes, since no Earth´s dust could reach altitudes where nuctilucent clouds form.
NLCs were first spotted after the eruption of Krakatoa volcano in 1885. After which they could be seen just after sunset worldwide. One sky watcher by the name of T.W. Blackhouse who´s recognized for their discovery stated that after the sun had set, on rare nights, he could witness thin threads of bright electric blue against the black sky. Scientists back then assumed that it was a natural manifestation of volcanic ash; however, eventually the ash settled and the brilliant nights came to an end.
Since their appearance in 1885, noctilucent clouds have remained. NLCs have not only persisted, but spread, as well. One hundred years ago, they could only be seen at latitudes above 50°; however, in recent years, they have been spotted in Utah and Colorado at latitudes of about 40°. In recent years, noctilucent clouds are even brighter and they have been appearing even during the day.
It is not well understood how ice crystals form in the mesosphere, since the mesosphere is not only extremely cold -125° C (257° F), but also extremely dry-100 million times drier than the Sahara desert. Krakatoa may have sent volcanic ashes to the mesosphere back in 1885; however, that doesn´t explain the formation of nuctilucent clouds in the present. It is thought that the source of nucleic particles could be the debris from comets and asteroids which are constantly sweeping the Earth´s orbit.
In addition, the apparition of nuctilucent clouds in the summer could be due to the fact that the upwelling winds transport moisture during the summer from the lower atmosphere to the dry environment of the mesosphere. It is believed that global warming could be a factor for the spread of noctilucent clouds, since greenhouse gases, besides warming the Earth´s surface, lower the temperatures at higher altitudes which are essential to the formation of ice crystals in the mesosphere.
The Aeronomy of Ice in the Mesosphere (AIM) satellite launched by NASA in 2006 is exploring nuctilucent clouds. Aim will monitor the formation of NLCs to better understand their connection to climate change. The main objective of AIM is to help scientists understand the nature of NLCs and their relation to climate change. In addition to monitoring environmental conditions, AIM will take wide angle photos; measure temperatures and chemical abundances of clouds; and the size of particles within them.
