Seeing Breath

It’s not cold out, it’s raining, our windows are open, and when we breathe out, we can see our breath, just like on a cold day. What’s happening?
- Jess (age 18)

It may not be cold out, but it’s still probably colder out than inside your body. Your breath is saturated with water vapor, meaning that if there were any more water molecules in it they would start to condense into droplets. If the breath cools down a little, that makes it easier for water to stay in liquid drops rather than in separate molecules, so visible drops start to form. On a dry day, it may be that by the time the drops would have formed, so much water has diffused out into the dry air that there’s not enough left to form liquid. On a wet day, almost as much diffuses in as out.
All these processes are governed by the laws of thermodynamics, which determine whether the liquid is stable in the presence of some concentration of gas molecules at some temeprature.

Mike W.

(published on 10/22/2007)