Okay, you could be easily forgiven for wondering what the heck you are looking at.
Early this morning, as in 0300 early, we were en route from Paris to Helsinki, just south of Stockholm at 39,000 feet. At exactly the same time as I was mentally formulating the question, my First Officer asked "Do you know what you are looking at?"
"Yeah, Noctilucent clouds."
Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. They consist of ice crystals and are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between 50° and 70° north and south of the Equator. They are visible only during local summer months and when the Sun is below the observer's horizon, but while the clouds are still in sunlight. Recent studies suggests that increased atmospheric methane emissions produce additional water vapor once the methane molecules reach the mesosphere - creating, or reinforcing existing noctilucent clouds.
They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 km (47 to 53 mi). They are too faint to be seen in daylight, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in Earth's shadow
In an absolute sense they are rare, and the very restricted observation conditions make seeing them far rarer still; this was a first for me. (As an aside, it doesn't hurt my Captain credibility when the FO asks a question he expects to be a stumper, and I just happened to have the correct answer right there. The big question is how it is that I was able to, within three-quarters of a second, use tenuous visual information to retrieve a term I had read at least a half dozen years prior. Humans do this all the time; it is extremely difficult to imagine AI ever managing it.)
Unfortunately, the camera in my now ancient iPhone 5S doesn't really do the scene justice -- there were patterns and structure that disappear in the noise.
Also worth noting is that this picture clearly shows a very sharp boundary between between the Troposphere and the Stratosphere, and, in so doing, also reveals the Earth's curvature.
More of the noctilucent clouds, with Stockholm in the foreground.
You have no idea how much I had to pay that damn seagull to bomb my photo.