Noctilucent Clouds

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Noctilucent Clouds

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Clouds of unknown composition which occur at great heights, 75 to 90 kilometers. They resemble thin cirrus, but usually with a bluish or silverish color, although sometimes orange to red, standing out against a dark night sky. Sometimes called luminous clouds. </dd>
These clouds have been seen rarely, and then only during twilight, especially with the sun between 5 and 13 degrees below the horizon; they have been observed only during summer months in both hemispheres (between latitudes 50 to 75 degrees N and 40 to 60 degrees S), and only in some parts of these latitude belts. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use