What is precipitable water vapor in millimeter-wave radioastronomy and how is it measured? Looking through ALMA on-line documentation and articles, the quantity PWV (precipitable water vapor) is a central theme. I have two linked questions.


What (actually) is precipitable water vapor, and is there a "non-precipitable" component also? Why not just call it "water vapor"?
How is it actually measured? It seems to be monitored by radiometers, but I don't understand how that can be used.


I usually see numbers of the order of 5 millimeter to 10 millimeters for very high altitude locations like Atacama. I assume this is the column-integrated amount, expressed in units of liquid, is that right?
As defined quite clearly, with equations, here

The total atmospheric water vapor contained in a vertical column of
unit cross-sectional area extending between any two specified levels,
commonly expressed in terms of the height to which that water
substance would stand if completely condensed and collected in a
vessel of the same unit cross section.

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There's, for example, an ALMA paper which discusses measurements and comparison with meteorological stations.