Definition
Rare, very high-altitude clouds that form near the mesopause, roughly 47 to 56 miles (75 to 90 km) above the Earth's surface. They are composed of tiny ice crystals and are visible only during deep twilight, when the observer is in darkness but sunlight still illuminates the clouds from below the horizon. They typically appear as thin, wispy, electric-blue or silvery sheets seen at high latitudes during summer.
Plain English
Very thin, glowing clouds that form so high in the atmosphere that they catch sunlight long after the sun has set on the ground. They look like silvery-blue streaks in the sky during deep twilight.
Context Anchor
Seen in aviation weather study and night-sky observation, especially when identifying unusual cloud types that are not part of normal flight-level weather.
Derivation
From Latin nocti- ('night') and lucent ('shining' or 'glowing'). The name literally means 'night-shining,' which describes exactly what they do — glow in the sky after dark.
Why Pilots Care
These clouds form far above the operational ceiling of any aircraft and pose no flight hazard. Pilots care mainly because they are a recognizable upper-atmosphere phenomenon worth identifying correctly and not confusing with weather-relevant cloud types.
Grounding Statement
Imagine standing outside about an hour after sunset in summer at a northern latitude. The sky overhead is dark, but thin, glowing silvery-blue streaks are still lit up high above — sunlight is reaching them even though it has long since left the ground.
Intuition Check
Do not read “noctilucent clouds” as ordinary clouds that simply appear at night. The key idea is that they are extremely high clouds lit by the Sun when the lower sky is already dark.
Example Sentence 1
On a clear summer evening in Alaska, the pilot pointed out faint noctilucent clouds shimmering near the horizon long after sunset.
Example Sentence 2
Noctilucent clouds appeared above the aircraft while the crew flew at cruise altitude in summer twilight.