What are the highest clouds in Earth's atmosphere, forming at about 80 km and glowing at night?

The highest clouds in Earth's atmosphere, forming about 80 kilometers up and glowing at night, are noctilucent clouds.

Their name means “night-shining,” and they appear after sunset or before sunrise because the lower atmosphere is dark while the clouds remain high enough to catch sunlight. They form in the mesosphere, near the cold summer mesopause, where water vapor freezes into crystals only tens to hundreds of nanometers across.

Noctilucent clouds are usually seen at high latitudes, roughly between 50° and 70° north or south. From Earth they look like delicate blue or silvery ripples, streaks, or wave-like filaments. Scientists studying them also call them polar mesospheric clouds when observing them from space.

They are often confused with nacreous, or polar stratospheric, clouds. Nacreous clouds occur much lower in the stratosphere and are associated with winter polar conditions, while noctilucent clouds are the loftiest clouds and generally appear during polar summer. Their visibility and brightness may also vary with atmospheric dust, methane, water vapor, and the solar cycle.