Re: What keeps clouds up?

[William Beaty <billb@eskimo.com>]

On Sat, 22 Mar 1997, Leigh Palmer wrote:

> Well, David Bowman's answer reveals some of the complexity of this
> particular problem, but one has to recognize the superior simplicity
> and conceptual accessibility of Hewitt's answer to the question. Is
> it worth sacrificing those advantages just because that answer is
> fundamentally incorrect and devoid of insight into the true nature
> of the physics?

Heh!

In rereading Hewitt's column, I see the possiblity that he might have been
intentionally avoiding all this stuff we're discussing. 

   We know that water vapor is less dense than air.  But we know that a
   water droplet in air is much denser than air, and the gravitational
   force pulling it down is much greater than the bouyant force pushing it
   up.  So why is it that all the water droplets in a cloud don't fall to
   the ground?

The wording of this question makes it into a "trick question," in that it
only asks about the downward falling of individual droplets WRT the air. 
He doesn't ask why clouds stay up, he asks why cloud *droplets* stay up. 
Hewitt's answer however causes problems.  If he had only pointed out that
the droplets are so small that they fall slowly, then his answer would
remain in the same limited domain of the question, and both would be too
narrowly focused to attract complaints.  But his answer also mentions
updrafts, and that immediately leads to the more general question as to
"why do clouds stay up."  His answer doesn't cover this question at all. 
Maybe it wasn't intended to. 

But as you say, students usually ask the latter question.  The answer
about individual droplets is far too incomplete to aid in a classroom
discussion.


Other topic: clouds that *do* fall.  If a cloud forms only because of
removal of "heat" (cough cough), and no pressure drop is involved, then
the air between the droplets will not heat and expand, and that cloud
*should* be much denser than the surrounding air.  It should pour downward
like a liquid.  I've seen this from above on an airliner.  At night the
earth's surface cools from radiation loss to space, and the air cools by
conduction to the earth.  Rather than the air temperature falling,
condensation occurs.  The cooling is driving the condensation, rather than
condensation causing temperature rise as with cumulus, etc.  The
low-density water vapor is removed, and turns into suspended liquid
droplets, so the net density of the resulting cloud increases.  Fog is the
result.  Viewed from above, this "radiation fog" can look like a pool of
milk in a valley between hills. 

The interesting thing I observed from an airliner was that a large pool of
fog was *pouring* from a valley through a gap between two hills and
spreading over a large flat landscape like a liquid.  The observable
structure of the flow, the little bits of turbulence, etc., looked
*exactly* like we see when white misty CO2 is poured across a table.  The
upper surface of the fog displayed slow moving gravity waves, just like a
pool of CO2 fog does. Fluid dynamics is neat!  Huge flow structures miles
across can be totally identical to tiny ones on a tabletop. 

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