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I wonder how laundry, which you hang inside or outside on a laundry rack, clothesline or something similiar becomes actually dry.

Water turns gaseous at around 100 C° (depends on the sea level of course) but undeniably the air temperature either inside or outside is not even remotely close to the 100 C°, especially not where I live in central Europe.

So, how do the water molecules actually get out of the laundry?

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No, water doesn't only turn into a gas at 100 °C. Every liquid has a vapor pressure dependent on its temperature. 100 °C is only the special case where the vapor pressure equals atmospheric pressure. Once that point is exceeded, bubbles form in the liquid, which we call "boiling".

Evaporation happens without boiling because the vapor pressure is non-zero. This process is slower because there is less pressure "forcing" the water vapor into the air.

Since the process is bi-directional, it also matters how much water is already in the air. Think of each molecule on the surface of the water having probability of detaching from the liquid and diffusing into the air. The higher the vapor pressure relative to the ambient pressure, the higher this probability. However, water molecules in the air also have a probability of condensing.

When there are few water molecules in the air, more will evaporate into the air than the other way around, and the clothes will dry. If, however, the air is humid enough and the clothes cool enough, water molecules in the air actually have a higher probability of condensing onto the clothes than they do evaporating from the clothes. In that case the clothes will actually get more wet. This phenomenon is commonly called "dew".

In typical situations of clothes on clothesline on a sunny day, the equilibrium reached where the same number of water molecules evaporate from the clothes as condense on them is what you call "dry". Even "dry" clothes in typical conditions still contain significant moisture, but not enough for us to feel.

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  • $\begingroup$ I would like to mark your reply as the accepted answer but if you could provide a reference/link/source to circumstantiate your reply, that would be great. :-) $\endgroup$ – Bruder Lustig Jan 3 '18 at 17:05
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    $\begingroup$ @Brud: I'm not sure what you want. Vapor pressure and the mechanics of dew are all very well known. I learned this stuff so long ago I don't remember where. Surely you can find a suitable reference with a little digging around on the net. $\endgroup$ – Olin Lathrop Jan 3 '18 at 20:36

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