Why does exhaled water vapour disappear quickly? If we exhale in the cold environment, water vapour condenses into little droplets, so we can see it. Why does this condensed cloud disappear so quickly? What happens to it ?
 A: I guess the water diffuses into a larger volume, the water content per volume drops below the saturation level, and the droplets evaporate.
A: Moist exhaled air is at body temperature; when it begins to mix with
cooler ambient air, the low partial pressure of water at ambient temperature
means that the moisture will form droplets (because it started at higher
concentration/partial pressure than cool air allows).
The droplets, then, diffuse with your exhaled breath into a larger volume
than your lung volume, and those droplets are in cool and dry ambient
air.   So, they evaporate.  Slowly at first, when volume per drop is high, and 
surface area is relatively small because of square-cube discrepancy, but
faster when the droplets are smaller.
One liter of air at body temperature and 95% relative humidity contains
0.04 grams of water.   At 0 C, one liter of air at 99% relative humidity
contains 0.005 grams of water, so you might expect up to .035grams of
condensation.   At 0C, and 50% relative humidity,  it takes 14 liters of
cold air mixed with your exhalation to evaporate that
condensate.
