|MadSci Network: Biophysics|
The short answer is, at the same temperature as your breath.
The long answer is:
Condensation of water vapor into aerosoled liquid (which is what happens when you see your breath in cold weather) is always dependent upon 2 main factors:
1. The ambient pressure acting upon the water vapor.
2. The temperature at which the water vapor is is at.
When water is at a particular temperature and at particular pressure, it will take on either a solid, liquid, or gaseous state.
When gaseous water is dissolved in air (20% O2, 70% N2), the question of threshold condensation is most conveniently summed up in the term/measurement of relative humidity.
The relative humidity of air conveniently excludes questions of temperature/pressure calculations and abstacts the question of the less rigorous inquiry "how much water is in the air"
By incorporating all the messy details of pressure/temperature thermodynamical calculations into this relative humidity measurement, the practical answer of 'when does the air yield up liquid water" is summed up by asserting that any water in excess of 100% relative humidity will cause visible condensation.
If a sample of gaseous water (possibly dissolved in another gas) is brought to an environment where that sample's relative humidity changes from its source environment of something less than 100% to its desination environment of something more than 100%, then you will see liquid H20 condensing into the fog-like form you see on a cold day from your breath. Note that if the relative humidity is very high, you may see such fog even at a relatively higher temperature, like in the 50degF range.
Try the links in the MadSci Library for more information on Biophysics.