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From: platt@emory.uucp (Dan Platt)
Newsgroups: sci.misc,sci.physics
Subject: Re: Question about relative humidity:  Why is it not always 100%?
Message-ID: <2212@emory.uucp>
Date: Fri, 28-Aug-87 04:14:34 EDT
Article-I.D.: emory.2212
Posted: Fri Aug 28 04:14:34 1987
Date-Received: Sun, 30-Aug-87 01:03:30 EDT
References: <1067@hoqax.UUCP>
Reply-To: platt@emory.UUCP (Dan Platt)
Organization: Math & Computer Science, Emory University, Atlanta
Lines: 46
Xref: mnetor sci.misc:469 sci.physics:2105

In article <1067@hoqax.UUCP> young@hoqax.UUCP (HUH) writes:
>
>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.
>
>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?

In answer to this, there are lots of things that happen to air as it
moves around.  Air moving over water may have a high vapor pressure
of water.  If the air rises and cools, the water condenses and it
rains.  If this air sinks, or is warmed, then the relative humidity
of the air drops.  Around low pressure zones, the air rushes into
the middle to fill up the low pressure, and the air rises in the 
center (generally) which is one of many reasons why low pressure 
zones are so often spawning storms.  On the other hand, high
pressure zones are composed of air moving away from a high pressure
area, with falling air replacing the air that moved away.  This air
has a tendency of being dry (unless it's bringing air from over
water like the Bermuda high does in Georgia -- then the air is humid,
and the heating of this air by the ground causes enough lifting to
make local thunderstorms, but little cooling).  On yet a grander
scale, there are bands called Hadley cells in which the air rises,
travels along a toroidal shape (like a doughnut), such as what
happens in the tropics.  The hot air from the Equator rises, drops
the moisture, moves north (or south), sinks around the tropic lines,
making a really dry air mass which goes south (or north) taking
what water it can find back to the equator.  This is why there
are jungles around the equator, but most of the tropic circles
run through deserts (like the Sahara, Gobi, Australia, Kalahari, etc).

The reason for all of this mixing and turbulence, and the resulting
failure for the atmosphere to be in equilibrium with the water
is that the presence of the water vapor allows a great release of
energy as the water condenses (latent heat of vaporization).  This
allows for the formation of heat engines (called Huricanes) in the
tropics as an extreme case, but also explaines why the atmosphere
turns over so violently around thunder storms.  There's a lot
of energy represented in the lifting of vapor laden air -- which
is released upon condensation.

Dan