original_query: What are noctilucent clouds?
query: What are noctilucent clouds?
answer: Noctilucent clouds, also known as night-shining clouds, are tenuous cloud-like phenomena in the Earth's upper atmosphere, composed of tiny water ice crystals. They are the highest clouds in the atmosphere, located in the mesosphere, and are visible only during astronomical twilight when the lower atmosphere is in shadow but the clouds are still illuminated by the sun.