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From: KFL%MIT-MC@sri-unix.UUCP
Newsgroups: net.physics
Subject: none
Message-ID: <15746@sri-arpa.UUCP>
Date: Thu, 19-Jan-84 22:11:00 EST
Article-I.D.: sri-arpa.15746
Posted: Thu Jan 19 22:11:00 1984
Date-Received: Sat, 21-Jan-84 07:17:30 EST
Lines: 33

From:  Keith F. Lynch <KFL @ MIT-MC>

  Water does not 'dissolve' in air.  This is a common misconception,
that water vapor is in the air because the air has some 'carrying
capacity' for water, and that the relative humidity is the proportion
of that capacity that the water vapor is currently filling.
  Water (like all substances) has a 'vapor pressure'.  Water's vapor
pressure varies greatly with temperature.  Water boils when it becomes
hot enough that its vapor pressure exceeds the pressure of the air (or
whatever) that is around it.  This explains why water boils at a lower
temperature at high altitudes, and will boil at any temperature in a
vacuum.
  Thus the 'carrying capacity of the air' depends strongly on temerature
but not at all on the pressure or on the composition of the air (excluding
gasses which chemically react with water, such as chlorine) and would be
excatly the same even if there was no air!

      Temp (C)	Vapor pressure (mm of Mercury)

	-20	   0.8
	  0	   4.6
	 20	  17.5
	 40	  55.3
	 60	 149.3
	 80	 355.1
	100	 760.0  (1 atmosphere)
	120	1489.1
	140	2710.9

  Note how rapidly the vapor pressure changes.  Also note that the vapor
pressure of ice is NOT zero (i.e. ice can and does evaporate (slowly)).

								...Keith