I live in an apartment close to San Francisco, California. I take a shower every morning and then hang up my wet bath towel over the sliding glass door to “dry”. Every morning my bath towel is bone dry. I assume the water in the towel simply evaporates into the air with 24 hours. There is no fan running in the bathroom and the temperature is usually around 70 degrees. The relative humdity outside is currently at 74 percent… hardly arid.
So I understand how a towel dries if you hang it outside in the sun and the towel warms up and that energy "drives’ the water out. but how does this exact same mechanism work in a darkened bathroom? There are no windows or heat source to provide energy. The air somehow just absorbs the water that was in the towel regardless of how warm or cold it is outside, or what the humidity is.
What force is compelling the towel to lose it’s moisture?
The towel is blissfully ignorant. Some molecules of water take on heat from the surrounding air and then get energetic enough to launch out into the air. This process continues until the towel’s level of moisture comes to equilibrium with the surrounding air. (You may have noticed that your shower enclosure also dries out.)
Wow! I never would have thought there was enough heat in the surrounding air to completely dry a wet towel, but I guess that must be the case. Mystery solved!
That’s because you’re thinking of the heat as how hot or cold it feels to you, when even 0 degrees Celsius can be described as 273 Kelvin. Even though something is cold to the touch, it’s still holding heat energy.
ETA: It’s more of a vapor pressure equilibrium phenomenon than a heat phenomenon; more heat just makes it happen faster. The towel will dry at 1 degree Celsius, it will just take a* lot* longer.
To elaborate a bit: Most folks know that the molecules of a thing (water, in this case) are always moving, and that the amount they’re moving depends on the temperature. This is true. But they’re not all moving at exactly the same speed. There’s some average speed for any given temperature, and most of the molecules are close to that speed, but a small number of the molecules are moving much faster, or much slower. In fact, in the theoretical textbook case, there’s no limit on how fast a molecule can be going, at any given temperature, but the further you get from the average speed, the less likely it is. There’s a particular distribution of speeds, called a thermal distribution, that things naturally tend to, if left to their own devices.
Now, there’s also some threshhold of speed for a water molecule to stay in the liquid (be it in your towell, or in a glass, or whatever). Any molecule going slower than that will stay in the liquid, and any molecule going faster than that will escape from the liquid and become part of a gas. But remember from above: There are always going to be a few molecules in the liquid that are above that escape threshhold. So those ones escape. But without those molecules at the high-speed end, the distribution of speeds is no longer a thermal distribution. It’ll drift back into a thermal distribution, but when it does, it’ll be at a slightly lower temperature (since it lost molecules exclusively from the high-energy end of the distribution). Now, it’s a thermal distribution again, and so there are again a small handful of molecules at the high end, and the process repeats. Eventually, most of the water will be gone.
Now suppose you have the same towel, but you start at a higher temperature. There’s still a threshhold energy for any given water molecule to escape from the towel, but that threshhold is closer to the average energy of the particles. So there are more particles above that energy, and so at any given time, you have more water molecules escaping from the towel. Thus, evaporation goes quicker, at higher temperatures. If you raise the temperature so high that the average energy of the molecules is at the threshhold temperature for escaping from the liquid, then evaporation occurs extremely rapidly, and you have what we call boiling.
Even frozen things can lose moisture to the air. That’s how freezer burn happens - water molecules escape from the surface of frozen food into the relatively dry air of the freezer.
And then there’s the surface area of the towel. It’s much more than the 2’ x 4’ area that is covered when you lay it out flat. I can’t quote you an exact number, but every one of the little loops of thread that are so good at absorbing the water from your body also do double duty to release those same water molecules back into the air.
WAG that the equivalent surface area of a totally flat surface would be closer to 50 ft/sq than to the 8 of the basic towel.
>And then there’s the surface area of the towel. It’s much more than… every one of the little loops of thread that are so good at absorbing the water from your body also do double duty to release those same water molecules back into the air.
This would only be relevant to the extent that the airspace very close to the fiber surface is the limiting factor. It’s liklier that there’s a more important limiting factor close to the surface of the entire towel. A good test of this would be whether the towel would dry more slowly if it were wadded up, which maintains the little loops area and reduces the close to towel area.
The fibers are important in another way, which makes it harder to dry the towel. Wherever there are tiny cavities, such as where two cylindrical bits of fiber touch and have a vanishingly fine crack between them, water can condense in that space if it has an affinity for the substance of the towel (which is probably mostly cellulose). If the fibers are fine enough the towel could actually pick up a significant bit of weight from humid air.
