In cold weather, we get water vapour coming out of the mouth as we breathe. What is that called?
This is simply called "breath", with no special word for the visible form. We often call this phenomenon "seeing one's breath":
It's so cold I can see my breath!
I would call that steam (definition 2b from m-w.com: the mist formed by the condensation on cooling of water vapor); even though it is most commonly associated with boiling, the word can be applied to any vapor created due to a heat differential.
(one example from the definition page: Their breath steamed the windows.)
comet-tail n. the tail of, or a tail like that of, a comet.
a1769 W. Falconer Descr. Ninety-gun Ship (R.), Its huge mast..From which a bloody pendant stretch'd afar Its comet-tail, denouncing ample war.
Why not call it
the breath condensing into mist
It was so cold that I could see my breath condensing into mist.
an opaque mass of something in the atmosphere.
protected by user140086 Apr 28 '16 at 14:33
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