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Re: condensation

To: Brust <roethler@cadetmail.uscga.edu>
Subject: Re: condensation
From: "W. R. Gibbons" <gibbons@northpole.med.uvm.edu>
Date: Thu, 21 Nov 1996 18:51:39 -0500 (EST)
On Thu, 21 Nov 1996, Brust wrote:

> I think the reason condensation occurs is a temperature difference on
> either side of the glass.   The reason it clears up quicker if you have
> your window down or you blow cold air onto the window is that both sides of
> the window are the same temperature.  When you blow hot air onto the
> window, you must blow enough hot air to heat the window all the way
> through.  This is why back windows with those electric heat gizmos embedded
> in the glass clear the window so fast-- it is easier for the heat to get to
> both sides.
> Correct me if I'm wrong....

Interesting idea, but I don't agree.  Condensation occurs when the 
surface of the glass is cold enough to condense water out of the air 
that's in contact with it.  It is the relative humidity thing again.  

Warning:  former physics teacher is cranked up:

The amount of water vapor air can hold is a function of temperature.  As 
temperature increases, the absolute amount of water vapor a given volume 
of air can hold increases.  The relative humidity is a measure of how 
much water vapor is present per unit volume of air, relative to how 
much water vapor the air can hold at the existing temperature.  A 
relative humidity of 50% signifies that the actual water vapor content 
is half what the air can hold at the existing temperature.

Because the absolute amount the air can hold increases with temperature,
it follows that if you take air with a certain amount of water vapor per
unit volume and cool it, the relative humidity increases.  It increases
because the actual water vapor content does not change, but the amount the
air can hold decreases.  If the air is cooled until the relative humidity
reaches 100%, water will condense into visible drops.  The temperature at 
which this happens is called the dew point.  When you see the tv weather, 
they may say that the temperature is 50 degrees, and the dew point is 45 
degrees.  This means the air is nearly saturated with water at present, 
and if the temperature falls to 45 degrees it will be completely 
saturated--you will have fog.

Fog forms on windows when the surface temperature of the window is cool 
enough to drop the surrounding air temperature below the dew point.  So 
let's say the above conditions hold--temp is 50, dew point 45, and you 
get into your MG and start driving.  You have the top up, and your 
breathing is sufficient to increase the amount of water vapor in the air 
inside the car.  Pretty soon, the air has so much moisture in it that the 
dew point is now 50 degrees, same as the surface temperature of the 
glass.  You get fog.

It isn't the difference in the two sides of the glass, but how cold the 
glass is and what the relative humidity of the air in contact with the 
glass is.  In MO, where I once lived, a common problem in summer is fog 
on the outside of the glass because the A/C in the car is cranked up so 
high that the outside of the glass is cold.

A/C's are natural dehumidifiers--they can't help it.  If the air is 
humid, and you blow it across the cooling coils, water condenses out.  If 
you want to make a dehumidifier, you just make a little refrigerator, 
blow the room air across the cooling coil, and collect the water.

Teach mode off.

   Ray Gibbons  Dept. of Molecular Physiology & Biophysics
                Univ. of Vermont College of Medicine, Burlington, VT
                gibbons@northpole.med.uvm.edu  (802) 656-8910


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