NASA's Astronomy Picture of the Day for October 27, 2026 featured noctilucent clouds photographed in early August above a field of green grass near Linköping, Sweden, with trees in dark silhouette along the horizon. The image is credited to V. Höfer, D. Večerník and B. Večerníková.

The agency's explanation notes that noctilucent clouds, the highest clouds, are more technically known as polar mesospheric clouds. They are normally too dim to see, but can become visible just after sunset during summer when sunlight from below the horizon illuminates them. Because sunset rises from the ground as Earth rotates, clouds high above can still catch sunlight after the ground darkens, allowing the blue network in the image to be seen well after nightfall.

According to the accompanying text, much about noctilucent clouds has been learned only in the past two decades, and how they form and evolve remains a subject of active research.