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From: young@hoqax.UUCP (HUH)
Newsgroups: sci.misc,sci.physics
Subject: Question about relative humidity:  Why is it not always 100%?
Message-ID: <1067@hoqax.UUCP>
Date: Thu, 27-Aug-87 11:34:21 EDT
Article-I.D.: hoqax.1067
Posted: Thu Aug 27 11:34:21 1987
Date-Received: Sat, 29-Aug-87 12:02:17 EDT
Organization: AT&T Bell Labs, Holmdel, NJ
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Xref: mnetor sci.misc:468 sci.physics:2100


There is something about relative humidity that I've been curious about for 
a while.  I am hoping that someone on the net could help me understand this.

From what I remember from my high school chemistry class, the relative humidity
(for a given location and time) is the ratio: 

	actual pressure of water vapor dissolved in the atmosphere 
	----------------------------------------------------------
	vapor pressure of water at the same temperature

where the vapor pressure is the equilibrium pressure of the vapor with its liquid
at a given temperature.

My question is, why is the relative humidity not always 100%?

Shouldn't the liquid water on the surface of the earth (which there seems to be
a plenty of) evaporate into the atmosphere and eventually reach an equilibrium,
thereby making the actual pressure of water vapor equal to the vapor pressure of
water?  (Think about a sealed jar partially filled with water.)  Can the concept 
of 'vapor pressure' be used in such a 'global' sense?  What other factors are 
there that influence the humidity?

Thanks in advance for any explanation.

Young Huh