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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 ?

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I guess the water diffuses into a larger volume, the water content per volume drops below the saturation level, and the droplets evaporate.

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  • $\begingroup$ +1 Just want to add that turbulent mixing of breath with cold ambient air is what causes rapid disappearance of cloud. If the flow were laminar you could have seen it a little longer. $\endgroup$ – Deep Dec 16 '16 at 5:40
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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.

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  • $\begingroup$ water evaporates despite the cold temperature? just because ambient air has low concentration of water vapour, I get it right? $\endgroup$ – Gleb Voronchikhin Dec 20 '16 at 9:26
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    $\begingroup$ Yes, the fog dissipates when the ambient air is unsaturated. That doesn't always happen, of course (we get a fair amount of fog in this area, including freezing fog). $\endgroup$ – Whit3rd Dec 20 '16 at 9:40

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