Noctilucent clouds form in the highest reaches of the atmosphere – the mesosphere – as much as 50 miles (80 km) above the Earth's surface. They're thought to be made of ice crystals that form on fine dust particles from meteors. ... Since atmospheric pressure decreases with altitude, the rising air expands.

What is the main reason for the formation of noctilucent clouds?

Noctilucent, or night-shining, clouds are the highest clouds in Earth's atmosphere. They form in the middle atmosphere, or mesosphere, roughly 80 kilometers (50 miles) above Earth's surface. The clouds form when water vapor freezes around specks of dust from incoming meteors.

Where do noctilucent clouds occur?

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.