A type of cirrus cloud resembling a spinal column or fish skeleton, hence the Latin vertebratus for vertebrae-like.
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These clouds form at very high altitudes (usually between 6000m and 14000m) and often indicate that stormy weather is coming. They usually start out as a smooth band of ice crystals that is then blown by crosswinds to create the fine streaks to either side of the central column. They can be straight or curved, like the one I saw today (in the image below, which is a very poor quality phone image, sorry)Image credit: ©PWL/Matt Fricker taken on 16 Nov 2014