Some contrails will spread vertically, part descending and part remaining, the effect can be like a curtain hanging from a curtain rod. I am guessing that this is due to larger particles, frozen or not, falling faster. Whether this is the case or not, what causes the descending "curtain" to cluster into regular puffs, so you get a sheet of vapor with very regular bumps in it? From directly beneath this regular repetition looks a lot like a backbone.
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