Re: Hot air rising

[Robert A Cohen <bbq@ESU.EDU>]

Boy, I see I've stumbled upon something here.  I'll start with William
Beaty's conclusion...

On Wed, 21 Jul 1999, William Beaty wrote:

> Is water vapor made of tiny wet droplets?  Is "moist air" wet or dry?  Is
> "steam" colored white or is it transparent & invisible?  Is fog a vapor?
> One path out of this morass is to temporarily stop using any terms which
> have multiple definitions.  Avoid "water vapor" and instead say
> "evaporated water" or "H2O gas."   Don't say "steam", say "cloud of
> droplets."   Avoid "moist air" and replace it with "air full of H2O gas."

This idea has a lot of merit.  I'll try to keep to this and not fall into
my meteorological jargon.

So, as for "mixing", I'm tempted to use "circulation"  instead of "mixing"
but I'll refer to the Glossary of Weather and Climate (1996), which states
that mixing is a "random exchange of fluid parcels on any scale."  I
believe my mistake was in identifying the scale in my original post.

> On Wed, 21 Jul 1999, John Denker wrote:
>
> Flow is not the same as mixing.

You are right; flow is not the same thing as large-scale vertical
exchanges of air.  Rather, flow is a part of large-scale vertical
exchanges of air.  (Vertical) transport (or flow) is required for a parcel
to experience adiabatic warming/cooling, but large-scale vertical
exchanges of air (of which flow is a part) is required for adiabatic lapse
rates (adiabatic vertical temperature gradients).

> Adiabatic cooling has everything to do with flow and nothing to do with mixing.

Adiabatic cooling will result from vertical transport because molecular
mixing is small and occurs over spatial scales of centimeters for the time
frames involved in vertical flow of the atmosphere.  It follows, then,
that adiabatic cooling/warming is also present in large-scale vertical
exchanges of air.

If a fluid flows, something has to take its place.  If we don't know
anything about what took its place then we can't say anything about the
vertical temperature gradient.  Only for large-scale vertical exchanges of
air can we say something about the vertical temperature gradient.

On Wed, 21 Jul 1999, Leigh Palmer wrote:

> Students often see physics as an exercise in studying a nonexistent
> universe. Their belief is that physics does not model the world they
> live in. Models that fail even the simple test of casual observation
> should be avoided! Simple physics doesn't explain everything. Even
> our complete physics cannot yet explain everything; indeed physics
> is incomplete. We should let on to our students that we know this.
> The real atmosphere problem is complicated. Any explanation of it
> should be heavily posted with caveats.

When the atmosphere is well-mixed (oops, I mean "characterized by
large-scale vertical exchanges of air such that conservative properties
are equalized"), the vertical temperature profile is observed to be
adiabatic.  See yesterday evening's sounding at Topeka, for example
(adiabatic from surface to about 1.8 km or so).  A simple model (as far as
I can tell) that is explained by a casual observation.

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| Robert Cohen               Department of Physics       |
|                            East Stroudsburg University |
| bbq@esu.edu                East Stroudsburg, PA  18301 |
| http://www.esu.edu/~bbq/   (570) 422-3428              |
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