Jane lay in the snow, looking at the stars.

[alpacinou]

What is a word/phrase I can used to describe the cloud of condensation formed over people's mouth when it's really cold?

Jane lay in the snow, looking at the stars. John was breathing fast, clouds of condensation forming over his mouth.

 

[Charlie Bernstein]

We usually just call it breath. It blows (or comes) out of your mouth. It doesn't stay there. He could see his breath would probably do it. You could elaborate:

- He watched his breath rise into the darkness.
- His breath rose into the air and disappeared.
- The steam of his breath rose and evaporated in the starlight.

Or something. The main thing is to always think about what's physically realistic.

 

[alpacinou]

Can I also use "vapor"?

Vapors of her cold breath floated in the freezing night air.

 

[Charlie Bernstein]

Can I also use "vapor"?

The vapor of her cold breath floated away in the freezing night air.

Again, don't make it linger. It doesn't. It vanishes.