Re: Money to burn?

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

On Mon, 28 Jun 1999, Herbert H Gottlieb wrote:

> On Mon, 28 Jun 1999 11:38:33 -0400 Robert A Cohen <bbq@ESU.EDU> writes:
> >
[snip]
> > Some words of caution regarding the "cloud in a jar" demo.  The
> > implication is that there are no nuclei in the jug until the smoke is
> > introduced.  This is not right, for even in smoke-free environments,
> > only very slight supersaturations are needed to form clouds.  In
> > fact, meteorologists assume that "any" supersaturation will form a cloud,
> > as there is an abundance of cloud condensation nuclei in the air.
> > Perhaps someone on the list can offer an explanation for why smoke is
> > needed at all then.  My guesses are that either (a) the water vapor
> > prefers the sides of the jar itself to the "natural" nuclei or (b)
> > that supersaturation isn't obtained at all and that the smoke (like salt)
> > can attract the water vapor at subsaturation (creating something more
> > like haze than clouds).
>
> According to the International Dictionary of Physics and Electronics, 2nd
> Edition 1961, Van Nostrand Company, p. 237......
>
> "CONDENSATION IN THE ATMOSPHERE. .........
> Nuclei upon  which condensation can take place are absolutely necessary.
> Non-ionized and pollution-free air will become up to 400% supersaturated
> before condensation occurs. On the other hand, clouds sometimes form
> before the air becomes 100% saturated. Nuclei upon which condensation
> begins at the approach toward saturation are highly hygroscopic and
> encourage condensation. Minute salt crystals, primarily sodium, magnesium
> and calcium chlorides, carbonates and sulfates, smoke particles and ions
> serve as condensation nuclei.  Lowering the temperature of air below its
> dew point, or adding water vapor beyond the holding capacity of air, is a
> second requirement for the formation of clouds."

Condensation nuclei are required for cloud formation at saturation
(really, just above saturation).  No argument there.

Apparently, however, there is some disagreement as to whether there are
sufficient cloud condensation nuclei in the jar BEFORE one adds the smoke.
I would argue that even before you add some smoke to the air in the jar,
the air in the jar has many, many condensation nuclei.  After all, do
clouds only exist where smoke is present?

Again, my concern about the demo is that it implies that "regular" air is
devoid of condensation nuclei.

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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              |
|                            **note new area code**      |
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