Noctilucent Clouds, or NCLs for short, are the highest forming clouds on Earth. They form at the very edge of Space, over 80km from the ground. They are very strange and mysterious clouds for a number of reasons. They only form in a band between 50 degrees and 70 degrees north or south of the equator. Also, they only form in the summer months. The clouds are faint and they are not visible during the daytime. They appear during twilight after the Sun has set. This gives them the name Noctilucent which loosely translates to ‘night shining‘ in Latin.
Nocilucent clouds appear as electric blue clouds and can be an amazing sight. The first recorded sighting was around 1885. Nobody is really sure whether they were there before or if they were new. There were spectacular sunsets around this time due to the eruption of Krakatoa in Indonesia in 1883. Some people believe it was because of this that people were watching the sky more and spotted the clouds. However, observations now show that the clouds have become more frequent in recent years. It is suspected that this is due to the rise in levels of the greenhouse gas, methane, in the atmosphere. When methane reaches the upper atmosphere, a complex process turns it into water vapor which aids the formation of NCLs.
For a long time the origin of the clouds was a mystery. As part of the research into them the Aeronomy of Ice in the Mesosphere (AIM) satellite was launched in 2007 to study NCLs. It is now thought that they form when summertime wisps of water vapor rise into the upper atmosphere, and crystallize on specks of dust from meteors that burned up there. The AIM satellite has found evidence of meteor smoke in the clouds which seems to back up this theory.
Ireland is very well placed to view NCLs. From late May right through to mid August, check the northern horizon between 90 mins and 2 hours after sunset or 90 min and 2 hours before sunrise. The first clouds of this year have been observer by the AIM satellite on 17th May so it is worth watching any evening from now on.