As shown in the figure:
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To give an analogy, children in a household always have the desire to run away from home, but those who have already left often want to come back. At any given moment, some are leaving, and some are returning. When the rates of both are equal, statistically it seems as though no children are leaving or returning, but in reality, it’s just that the numbers of those leaving and returning have reached equilibrium.
When more children leave, the density of runaway kids increases, which in turn raises the pressure for them to return home (it's tough out there). When the number of runaway kids reaches the maximum the city can accommodate (saturation), every child that leaves forces another to return, resulting in a dynamic balance in total numbers.
The above values are somewhat related to the number of adults and other small animals in the city, but the effect is minor and can generally be ignored. We can consider that it is mainly influenced by the number of children themselves.
(Moms, don’t come at me — this is just an analogy for better understanding, and doesn’t mean I’m heartless!)
Expressed in physical terms, vapor exerts pressure, and vapor pressure drives gaseous water molecules to return. We call the state of equilibrium “saturation,” meaning that the air has reached its maximum capacity for holding gaseous water molecules and can no longer accept more. Thus, for every molecule that vaporizes, another must be forced to liquefy. Unsaturated vapor pressure is lower than saturated vapor pressure (at the same temperature). It can be seen that the vapor pressure of water at saturation actually reflects the intensity of its evaporation tendency. Therefore, saturated vapor pressure can indicate the strength of a liquid’s volatility.
Additionally, vapor pressure increases with temperature and changes noticeably.
Continuing the analogy, if there are two neighboring cities, one with high density of runaway kids (high vapor pressure) and one with low density, what happens? Clearly, kids from the area with higher density (higher vapor pressure) will flow and spread toward the area with lower density (lower vapor pressure).
Applying these principles, we can understand how the breathability of a rain jacket changes. The reason why water molecules inside the jacket generally move outward is that the temperature inside is usually higher than outside, and the humidity is also higher, which naturally leads to higher vapor pressure inside the clothing. The overall tendency of the water molecules is to move outward, which is the principle behind breathability. The higher the external air humidity and temperature, the higher the external vapor pressure, and the lower the tendency of water molecules to move from inside the clothing to the outside, resulting in reduced breathability. It should be noted that the breathability of outdoor clothing refers not to the passage of air, but specifically to the permeability of water molecules, because its purpose is to keep the body surface and the under-layers dry.
From the above analysis, it can be seen that the factors affecting the breathability of water molecules (assuming the fabric remains unchanged) are mainly the temperature difference and humidity difference between the inside and outside. Those demonstration experiments where air pressure is applied to one side of the fabric to show that it can pass through and thus prove good breathability are actually misleading. Not to mention whether the human body can provide such high pressure inside a rain jacket, even if it could, it would affect other gases, not water vapor molecules. This is also why, in warm and humid southern environments, you may find that the breathability of a rain jacket is very poor. In fact, rain jackets are best suited for cold environments, where there is a large temperature difference between the inside and outside, and the cold air is relatively dry, leading to better breathability. Of course, excessively low temperatures may cause frost to form on the inside of the rain jacket, which also affects breathability—but that’s another issue for further discussion. But you can think of it simply this way: when sweat vapor freezes on the inner side of the jacket, it actually helps keep the inner clothing and skin dry by minimizing the impact of moisture on the inner layers (the insulating layer), reducing the effect of humidity, similar to the principle of low temperature and dryness.
Another misconception is the so-called “waterproof and breathable.” Indeed, there is waterproof performance, and there is also breathable performance, but whether they can coexist at the same time is debatable. When it rains, the humidity outside the rain jacket is extremely high. Based on vapor pressure analysis, breathability can basically be ignored at this time. In the rain, the function of a rain jacket is essentially that of a raincoat, and its ability to remain continuously waterproof is certainly not as good as that of a plastic poncho. Therefore, my personal view is: the key value of a rain jacket lies in its windproof and breathable properties, mainly suitable for cold, snowy environments. As for waterproofing, it’s enough if it can handle short periods of light to moderate rain—don’t expect too much.
The same logic applies to hiking boots.
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