Noctilucent Clouds (NLC) occur at extremely high altitude, about 50 miles (80 to 85 km), and glow at night.

How high up are noctilucent clouds?

Noctilucent clouds are composed of tiny crystals of water ice up to 100 nm in diameter and exist at a height of about 76 to 85 km (249,000 to 279,000 ft), higher than any other clouds in Earth's atmosphere.

Where are noctilucent clouds found?

Noctilucent clouds are the highest clouds on Earth, and are located in the coldest part of planet Earth, the high-latitude summertime mesopause region (80–90 km). Composed of water-ice particles, they are nucleated by meteoric smoke particles.