Imaging NOCTILUCENT CLOUDS
There’s a lot to be learnt about this eerily illuminated nocturnal event
Noctilucent clouds are high-altitude ice clouds that form 82km up in the mesosphere, lit up by the pre-dawn or post-sunset Sun to create ethereal displays that take many shapes and forms. The nature of NLCs is slowly being unravelled through observational recording, much of it carried out by amateur astronomers, though the frequency and brilliance of displays is difficult to predict.
One particularly useful area of study involves creating time-lapse sequences to reveal how the clouds flow and evolve with time. These timelapses can be wide-field or magnified sequences concentrating on small, structured areas.
It’s useful to make a note of the structures in NLCs, as well as display brightness, which is typically done using a five-point scale:
1 Very weak, barely visible to the naked eye 2 Clearly visible but of low brightness 3 Clearly visible, standing out sharply against the background sky 4 Very bright and obvious even to casual observers 5 Extremely bright and able to illuminate objects facing the display Another feature of interest is how an NLC display develops through the night. If there is a display it normally occurs when the Sun is between 6° and 16° below the horizon. From the UK, displays tend to appear low above the northwest horizon 90-120 minutes after sunset, or above the northeast horizon a similar time before sunrise. If they’re extensive, displays can be seen moving from the northwest through north and into the northeast. Typically described as a low-altitude phenomenon from the UK, under certain situations NLCs have been seen to extend to high altitudes, even overhead.
The appearance of NLCs in any particular year cannot be predicted with accuracy. Links to external influences, for example the 11-year solar cycle, can only be established by vigilant NLC patrol watches in which negative sightings have just as much importance as positives.