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Old 10-02-2019, 10:05 AM
Napier is offline
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Join Date: Jan 2001
Location: Mid Atlantic, USA
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Originally Posted by guizot View Post
Water condensing on a "flat water surface"? What is that supposed to mean? How do you differentiate between the the water forming the surface itself and the water condensing? How does water condense on water? And why do we want to measure that?

This is not to say the explanations are wrong--just rhetorically faulty.
The point is that relative humidity is not relative to the maximum possible amount of water vapor, a common misstatement. What it is relative to is the equilibrium between humid air and a flat water surface, at which evaporation and condensation are both going on at equal rates. Surface curvature matters. If you have supersaturation, does that mean there is more water vapor than physically possible? Of course not.

Here is something from the Wikipedia article on "Kelvin equation":
"The Kelvin equation describes the change in vapour pressure due to a curved liquid–vapor interface, such as the surface of a droplet. The vapor pressure at a convex curved surface is higher than that at a flat surface. The Kelvin equation is dependent upon thermodynamic principles and does not allude to special properties of materials. It is also used for determination of pore size distribution of a porous medium using adsorption porosimetry. The equation is named in honor of William Thomson, also known as Lord Kelvin."

And this, from the Wikipedia article on "Relative humidity":
"The relative humidity (...) of an air–water mixture is defined as the ratio of the partial pressure of water vapor (...) in the mixture to the equilibrium vapor pressure of water (...) over a flat surface of pure water at a given temperature (...)." [I skipped over the fonts and formatting issues because they don't seem to paste right here, but the sentence is the first one in the "Definition" section.]

One more item, from the Wikipedia article on "Vapor pressure":
"Meteorologists also use the term saturation vapor pressure to refer to the equilibrium vapor pressure of water or brine above a flat surface, to distinguish it from equilibrium vapor pressure, which takes into account the shape and size of water droplets and particulates in the atmosphere."