NLC - Noctilucent clouds
Night shining clouds - or scientifically known as noctilucent clouds - are a celestial phenomenon that is mostly unknown to amateur photographers in our latitudes and was first described in 1885.
Their low level of awareness is very unfortunate, as their beauty can certainly compete with the much more well-known northern lights. As with the northern lights, the NLCs occur in the north, mostly from latitudes above 50 degrees. The further north you are in the Northern Hemisphere, the more likely and the more intense it is to occur. They only become visible after sunset, when the sun is at least 6 degrees and a maximum of 16 degrees below the horizon.
Basically, their visibility and occurrence is limited to the period between the last decade of May and the second decade of August. In our latitudes, their occurrence is usually limited to the period between the beginning of June and the end of July at the most. The peak in the occurrence of NLCs in Central Europe is in the last half of June and the first half of July, although from experience I can say that the peak in Luxembourg is usually between June 20th and June 30th.
They can still be observed at the beginning of July, but usually in a much weaker form. After sunset, NLCs can be seen in the northwest of the sky before sunrise, in the second half of the night, they can be seen towards the northeast in the sky. They are easily visible to the naked eye, but are often confused with polar stratospheric clouds / cirrus clouds. Normally, you can see NLCs up to about 20 degrees above the horizon. Here you have to try to recognize the structure.
But what exactly are NLCs and how are they created?
NLCs are collections of ice crystals located in the mesopause above the mesosphere at an altitude of 81-85 km. Since they are at such an enormous altitude, they are still illuminated after sunset or before sunrise, when the sun is 6-16 degrees below the horizon, they shine silvery blue and have a fine fibrous structure.
The ice crystals are so-called crystallization nuclei, the origin of which has not yet been precisely clarified. Scientists are now convinced that this is material released when meteors burn up. In the mesosphere, pressure and temperature differences occur particularly frequently due to the prevailing low density, which lead to strong atmospheric turbulence and currents. Gases rise from the lower atmosphere and transport water vapor from the stratosphere to the much colder mesopause.
These gases are quickly mixed by the atmospheric turbulence and thus condense on the crystallization nuclei (material from meteors such as dust particles). A relaxation of the gas mixture, which is caused by the rapid transport of air masses through convection in the presence of water, means that local temperatures of up to -140 degrees can suddenly occur. Since -85 degrees usually prevail in the mesopause, this is a temperature anomaly in a very limited area. This appears to be an important prerequisite for the presence of crystallization nuclei, which in turn is a prerequisite for the formation of NLCs. Noctilucent clouds can also be found in the southern hemisphere.
It should be noted that on the night of June 21st to June 22nd, 2019, the largest NLC display ever observed was seen. Here the NLCs were visible even in Luxembourg immediately after sunset and remained present practically the entire night until shortly before 4 a.m. Even here in Luxembourg, the display extended beyond the zenith into the southern night sky, something that has never been observed before in history.
My photos in my book Letzebuerger Ardennen - Magical Lights from that night were taken when I was looking for a good photo spot for the national holiday and I can say that they are absolutely unique as I was the only photographer at that spot that evening.
When it comes to camera settings for NLC photography, the same basics apply as for night photography in general.
A fast wide-angle or ultra-wide-angle lens with an aperture number of at least f2.8 or smaller should be used, the ISO should be between 800 and 1,600 depending on the brightness, and the exposure time should also be 4-10 seconds depending on the brightness.