These clouds occur over the high latitudes of both the Northern and Southern Hemispheres during their respective summer months at the very high altitudes of approximately 47 to 53 miles.

They are most visible during twilight, when the clouds are still illuminated by the setting Sun, while the ground is already dark.

Polar mesospheric clouds are also known as noctilucent or “night-shining” clouds—a property that is clearly visible in this astronaut photograph.

The clouds exhibit thin, wispy light blue forms that contrast with the darkness of space (image upper right).

Lower levels of the clouds are more strongly illuminated by the Sun and appear light orange to white. Clouds closest to the Earth’s surface are reddish-orange (image center).

The image was taken approximately 38 minutes after midnight Greenwich Mean Time (GMT), while the International Space Station was located over the southern Atlantic Ocean.

At this time of year, the Sun never sets over Antarctica, but rather traces an arc across the local horizon, allowing polar mesospheric clouds to be observed near local midnight.

Full story and image: NASA