The first noctilucent clouds of this season have been spotted from NASA’s AIM spacecraft. These beautiful midnight clouds illuminate the twilight sky of summer. If you see any be sure to report it at the NLC Observers website.
Noctilucent clouds are the highest clouds in the Earth’s atmosphere, located in the mesosphere at altitudes of around 85 km, and are visible only when illuminated by sunlight from below the horizon while the ground and lower layers of the atmosphere are in the Earth’s shadow; otherwise they are too faint to be seen. Noctilucent clouds are not fully understood meteorological concepts. Clouds generally are not able to reach such high altitudes, especially under such thin air pressures.
Every summer since the late 1970s, radars probing Earth’s upper atmosphere have detected strong echoes from altitudes between 80 km and 90 km. These altitudes comprise the “noctilucent zone,” where water vapor crystallizes around meteor smoke to form icy noctilucent clouds (NLCs). The first NLCs of the 2015 northern summer season were spotted by NASA’s AIM spacecraft on May 19th. The radar echoes have followed close behind.
The NLC season runs from late May until late July, but they can also be seen at other times of the year, but this is rare. The normal latitude zone for observing NLC displays from is latitude 50º to 65º with 55º to 60º being most favourable. North of about 60º, it does not get sufficiently dark during the middle part of the season, so they can be difficult to see. NLC displays can also be seen from the corresponding southern latitudes with the season being December and January rather than June and July. Also, as there are fewer landmasses most of the displays are recorded by ships in the area.
NLC can be observed in various ways, but undoubtedly the easiest way is just with the naked eye, as like aurora they can spread right across the northern horizon from north east to north west, and the altitude due north can be several degrees, with the star Capella frequently appearing to be embedded in them. NLCs vary in brightness and shape, more information can be found at Martin McKenna’s excellent website, Night Sky Hunter.
Photographing NLCs is relatively easy; you will need a tripod, first and foremost. Then you simply take a long exposure time of around 10 – 15 seconds, depending on the brightness and the F-stop of your camera lens. If you take photos over regular intervals you can build up a beautiful animation/video of the clouds in action.
It is thought that NLCs exist at a temperature minimum in the Earth’s atmosphere. If the NLC height were to change, it would indicate a temperature change in the structure of the atmosphere, so the height of a display could be a possible warning of global warming and other effects.
Original article can be found at the Northern Ireland Amateur Astronomy Society.