----- Original Message ----- 
From: "Erik Reuter" <[EMAIL PROTECTED]>
To: "BRIN-L" <[EMAIL PROTECTED]>
Sent: Wednesday, January 28, 2004 5:57 AM
Subject: Dewpoint questions


> Any amateur meteorologists here? I noticed an odd phenomenon in my home
> recently, and I can't come up with any ideas to explain it.

I think I can do it from the basic physics.  Condensing usually occurs when
the temperature of water vapor is lowered to the point where the relative
humidity hits 100% and then keeps dropping.  Since the relative humidity
can't go above 100%, water must condense, releasing heat, and slowing the
cooling.

Lets look at case 1, a mirror at 68 degrees, with very dry air.  You go in
the shower and the vicinity of the shower is now at, say, 90 degrees with
near 100% humidity.  There is mixing, with the hot water vapor from the
shower going through the entire bathroom.

Water vapor has a high heat capacity, so it quickly warms up the dry
atmosphere of the room.  Soon, you have 100% humidity at, say, 75 degrees,
while the mirror is still at 68 degrees.  Condensation occures.

Now, let us consider a room with 50% humidity.  The air doesn't heat up as
fast because it holds water vapor at 68 degrees.  Thus, the mirror has time
to raise its temperature, and the temperature differential between the
mirror and the air at 100% humidity is noticably smaller.  Thus, it doesn't
fog up.

Dan M.



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