Have you ever heard of shining or glowing night clouds? I had never before coming to Denmark, but they exist! But here is the trick, not everywhere...
This phenomenon, also known as noctilucent (from Latin, 'night shining') clouds, is usually observed at mid to high latitudes, between 50 and 65 degrees North and South from the equator. Nonetheless they can be seen at lower latitudes low on the horizon, but it is usually too dark. At higher latitudes, the summer sun never sets, making it impossible to see them.
These strange clouds are only visible during summer time, typically from late May to late July, because the Earth is at its farthest away position from the sun on its elliptical orbit, causing the upper layers of the atmosphere (mesosphere) to reach very low negative temperatures (about -120 degrees Celcius). The mesosphere is usually very dry and contains virtually no water, but in these conditions, extremely thin (<100 nm in diameter) ice particles form.
Noctilucent clouds only appear in a deep twilight, approximately one to two hours after sunset or before sunrise. At these latitudes, summer nights are bathed in a never-ending nautical twilight, making it possible to see the glowing clouds. They appear, because the sun, close under the horizon, projects its rays with a certain angle on the mesospheric ice particles, causing them to glow. Although the presence of tropospheric clouds is the key to producing this 'back-lighting' occurrence (see on the diagram below, credits: communitycloudatlas.wordpress.com)
Noctilucent clouds are elusive and tricky. They come in different shapes and intensities, depending on environmental factors, and they certainly cannot be forecast (more info on this cool website: click me). You will need clear skies and patience to be able to see them, but once you do, they are unmistakable. While all other clouds appear dark, these clouds glow in the night sky! If you want to go out and chase them, your best chance are on a clear summer night at latitude 50-65, between 11.00 p.m. to 3.00 a.m., look towards towards the North where the nautical twilight never disappears, and every once in awhile, they put on a majestic show.
Last night was one of them... As mentioned in a previous post, I had already seen them a week and a half ago. Since then, all the nights were clear and favorable, as if summer had permanently settled in over Denmark, to a point where my lawn had turned almost yellow from the drought! Unfortunately, none of the 10 nights or so were successful. I have shot some cool moonset time-lapses, which will be featured in my next film. Last night was the last clear night before the forecast predicted rain and clouds, but I hadn't planned to go outside before midnight, because I was finishing my show. ROOKY MISTAKE. As I peeked outside the window, they were there! I literally stormed out to the beach with my cameras and raced towards the small boat I wanted to feature in a time-lapse, and started shooting. I was so happy, but I was unable to catch my breath after racing along the beach.
As I was shooting, a very unexpected guest made its appearance. Wait... two guests. Two grey herons landed next to my shooting place and started foraging in the shallow waters of Høve strand bay, as if I wasn't here. They stayed for a while and I was able to capture a cool time lapse and some great shots. I love these situations, because they combine great biological and environmental phenomena!
NLC are fast-evolving clouds that appear in the twilight, so if you want to go out with your camera and capture them, you will need some specific settings. You don't want too large apertures, because they will cause too much vignetting with wide-angle lenses (which can be corrected in post-processing), plus the smaller the aperture, the sharper the image... I usually use between f/3.2 and f/5.6, but it will all depend on your lens and the other conditions. I try not to use ISO's higher than 400 because of the noise performances of my cameras, but you can try higher on yours. The exposure time depends on the two other settings, but keep in mind that NLC are habitually evolving fast, depending on the currents. I would choose a shutter speed of anywhere between 1 and 10 seconds, higher speeds causing your clouds to be blurry (still depending on your focal length). Needless to say that with these exposure times, you will need a sturdy tripod and a release cable. With all that in mind NLC are yours to conquer and gaze at.