Clouds that shine at night

In late spring and summer, unusual clouds form high in the atmosphere above the world's polar regions. As the lower atmosphere warms, the upper atmosphere cools and ice crystals form in meteorite dust and other particles high in the sky. The result is noctilucent or "night glow" (NLC) clouds, electric blue wisps that grow at the edge of space.

Composite image of the North Pole showing noctilucent clouds appearing in various shades from light blue to white, depending on density. of ice particles.

Noctilucent clouds are definitely the highest as they form at heights of up to 85 kilometers! at that point there is hardly any trace of air left in the atmosphere. Literally, you can consider the boundary between Earth and space (not counting things like the ionosphere or Earth's magnetic field)

Noctilucent clouds were first described in the mid-19th century after the eruption of the Krakatau volcano. Volcanic ash spread through the atmosphere, creating vivid sunsets around the world and triggering the first known NLC observations. At first, people thought they were a side effect of the volcano, but long after the Krakatau ash settled, the wispy, glowing clouds remained.