Final answer:
Noctilucent clouds are unique due to their high altitude, icy composition, and their visibility during summer months in high latitudes. They shine at night by reflecting sunlight due to their high altitude.
Step-by-step explanation:
Noctilucent clouds, also known as night shining clouds, are unique in several ways. Firstly, they are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers. Because of their high altitude, they are able to reflect sunlight to the Earth long after the Sun has dropped below the horizon, thus giving them their 'night shining' appearance.
Secondly, noctilucent clouds are made up of tiny ice particles. The cold temperatures at these high altitudes allow the water vapor present there to freeze around tiny dust particles from meteors, creating these icy clouds.
Lastly, noctilucent clouds are not a common atmospheric feature. They are mainly visible at high latitudes during summer months when the mesosphere is at its coldest and there's enough solar radiation to illuminate them.
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