Noctilucent clouds seem to glow after sunset because they are so high in the atmosphere that the Sun can still illuminate them after it has dropped below the horizon for someone on the ground. Tiny ice crystals scatter the sunlight, creating delicate blue or silver streaks.
The clouds are extraordinarily high
Noctilucent, or “night-shining,” clouds form in the mesosphere, roughly 80 kilometers above Earth. At that height, they can remain in sunlight after lower clouds and the ground have entered darkness. [1]
Tiny ice crystals scatter sunlight
The clouds consist of small ice crystals. They do not produce their own light; instead, they reflect or scatter sunlight toward observers beneath them. Their blue-white appearance stands out against the darker twilight sky. [2]
A rare twilight sight
They are most often visible in summer at high latitudes, when the geometry of twilight makes the upper atmosphere sunlit while the observer’s sky is dark enough to reveal the clouds. Their occurrence also depends on the very cold, dry conditions of the mesosphere.
Are noctilucent clouds glowing by themselves?
No. They appear luminous because high-altitude ice crystals are still lit by the Sun.
Sources and evidence
- NASA — Taking AIM at Night-Shining Clouds ↗
Explains that noctilucent clouds form in the mesosphere and their ice crystals reflect sunlight to create a blue glow.
- NOAA NESDIS — A Mesospheric Marvel: Rare Clouds Spotted via Satellite ↗
Describes noctilucent clouds as polar mesospheric clouds and discusses the rare ice-crystal conditions at their altitude.
Sources consulted: 15 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against linked institutional, medical, or scientific references. This article was prepared with AI assistance and has not received independent expert review.