Noctilucent clouds, also known as polar mesospheric clouds, are ice-crystal formations found in a region of the upper atmosphere called the mesosphere. They form at altitudes of 50 miles (80 km), right up near the fringes of Space, making them Earth’s highest clouds. Named with the Latin for ‘night shining’, they have a bluish glow as they catch the light when the rest of the sky is dark. The clouds are seen only during twilight hours when the Sun is below the horizon (between 6° and 16° below, to be precise). They are only visible during summertime, and from mid-to-high latitude regions of the world.
Over the past few decades, sightings of noctilucent clouds have become more frequent. They have been seen from more extensive regions, appearing further south in the Northern Hemisphere, for instance. It is possible the increasing frequency of this mysterious and ghostly formation is a consequence of our warming climate.
You can read more about noctilucent clouds and their possible connections to climate change in this BBC Sky at Night Magazine article.