Re: Money to burn?

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

On Sun, 27 Jun 1999, Herbert H Gottlieb wrote:

> [snip]
>
> CONDENSATION OF WATER VAPOR. A simple cloud chamber can be made from a
> gallon jug fitted with a one-hole stopper with a short piece of glass
> tubing.  Blow into the jug through the glass tubing to increase
> pressure.  Put finger over end of tube and pull stopper, suddenly
> reducing the pressure.  No cloud is formed.  If some smoke is introduced
> into the jug it provides nuclei about which water vapor condenses.
> Repeat the performance and watch the clouds form in the jug.
> stationary.
>
> [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).

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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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