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Banner cloud on the Matterhorn showing strong wind of cold dry air on one side and warmer humid air on the other producing the cloud in a banner shape

Banner cloud

Banner clouds can form on the side of a high or exposed peak and seem to stick to the mountain face like a banner, even on an otherwise cloudless day. There are several theories for how they form but in general strong winds of cold dry air from one side likely interact with warmer humid air on the other. Look out for them next time you're in the mountains.

Also see lenticular clouds, thunder clouds, and more clouds.

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