Andrew Gideon wrote:


But I'm still not seeing
how the concentration varies with the air's starting dewpoint. Is it that
a deeper cloud will accumulate more water at the top because the
condensation has been occurring over more altitude, and that water has been
rising?



The dewpoint is a (reasonably) exact measure of the invisible moisture
in the air.

If you start with a parcel at temperature 30, dewpoint 20, it contains
about 15 grams of water vapor for each kilogram of air. If lifted, it
will start to condense at about 4,000 feet.

If lifted all the way to 18,000 feet, it will have to lose about 10.5
grams into liquid per kilogram of air. And yes, as it continues upward,
the already-condensed water follows in the updraft.

The air following it, which has only been lifted to 10,000 feet, will
only have to lose about 6 grams into liquid for each kilogram of air.

So there is about 4.5 more grams of liquid water at 18,000 feet for
every kilogram of air.