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Is the concept of "saturation" of air by water "real"?
I have found an excellent website debunking common scientific misunderstandings. I still have trouble with this one...they say there that there is no inherent capacity of water vapor in air, rather it is either water vapor or water droplets depending on pressure and temperature, only. I understand their reasoning but it still seems intuitively that Houston in the summer vs Phoenix can have the same pressure and temp but the air in Houston will only support liquid water vs all in Phoenix will only be as vapor. What I mean is that Houston air is "topped out" or "saturated" (which is what they say is the wrong concept). Is that right? The site and discussion of the topic is called "Bad Meteorology"
http://www.ems.psu.edu/%7Efraser/BadMeteorology.html
In thermodynamics textbooks, in the chapter about phase changes, you might find an experiment described. In vacuum (usually, barometric vacuum - a long glass tube filled with mercury, turned upside down in a dish also filled with mercury; the mercury will go up only 760mm, anything above that is vacuum) you add water in small drops (in the above setup, insert them at the base of the tube; they will float to the top in the tube)
You'll notice that the water drops vanish almost immediately (they turn into gas), until a maximum quantity is reached. Anything added after this maximum will not turn into gas. You'll notice that the mercury column dropped, indicating the fact that the vacuum was replaced by a gas at a certain pressure. This pressure can be measured by the drop in the mercury column. Changing the geometry (column length and area, for example), you'll notice that pressure drop is the same at the same temperature, regardless of geometry. This pressure is called "saturating vapor pressure", and is a property of the substance itself (water in this case). If using a clean gas (nitrogen, oxygen or pure, dry air, for example), the process is the same; you still have the maximum water quantity which changes into vapors, the pressure change is the same (with temperature), but the vaporization process is slower.
What is called "relative humidity" is the actual pressure of water vapors in the air, relative to the maximum pressure at the given temperature. If the actual pressure of the water vapors pressure is equal to the saturating vapor pressure, we say that the air is saturated. But the maximum water vapor pressure is the same regardless of air pressure, at the sea level or on top of Mount Everest.
The reason they say that there is no inherent capacity of water vapors in air is probably because the "water vapor capacity" is not a property of the air, but of the water itself. To say that this is a property of the air it would mean that the quantity of water in air depends on the air quantity (which it does not), similar to dissolving salt in water (the quantity of salt in water DOES depend on the quantity of water - you can reach a maximum CONCENTRATION, not a maximum QUANTITY, as is the case with water.
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