Why? Because the respective altitude (80-90 km above ground) is coldest in summer such that ice can crystallize onto meteoric dust despite there not being much water in the air to begin with.
This raises some questions: Why do we even see these clouds? Should ice not be transparent?
Yes. Ice is virtually transparent. However, depending on the purity, thickness and structure of the icy particle at least some light is scattered or reflected. Less than a tenth of a percent of the incident sunlight is scattered by NLC, which means that the clouds are only visible under rather special circumstances.
The image below summarizes these circumstances. In order to see what little light there is one has to have night sky above the cloud, yet the Sun cannot be so far below the horizon as to no longer illuminate the icy particles.
In other words, NLC are essentially twilight phenomena.
It does not stop there. Why would it be so cold – in the summer of all times?
The temperature in the middle atmosphere is connected to pressure waves generated in the lower atmosphere, which travel upwards and lose most of their energy at NLC-altitudes. You can sometimes see these waves in fishbone-like structures in the NLC themselves.
The energy lost is used by nature to drive air away from the Summer pole, thus creating a large and almost permanent low pressure volume. In this volume, the temperature can drop to -140o C.
“Permanent” is the wrong word, strictly spoken. For when the seasons change the pattern of pressure waves changes, too, and the wind at NLC-altitudes changes direction, again via what is called “pressure wave driving”.
Do not ask me to explain pressure wave driving, by the way. The explanation lies in mathematics, and gaining knowledge by means of mathematics is a completely different story, which, I am sure, we will have to come back to eventually.
Alexander is a physicist, teacher and science communicator who is currently working at the Norwegian Centre for Space-related Education at Andøya Space Center in Norway. Even though, in his case, work and play do overlap, the content on this webpage is entirely private. You can follow Alexander on Twitter, Facebook and Google +.