Date: Fri, 28 Jun 2002 16:27:46 -0400
Reply-To: Ed McLean <email99@BELLSOUTH.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Ed McLean <email99@BELLSOUTH.NET>
Subject: Humid Air is less dense than Dry Air!!!
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Humid air is less dense than dry air and will consequently be less
effective at cooling a radiator.
Rain is a completely different story.
It's simple physical chemistry.
If you wish to be convinced I invite you to do a Google search for "air
density" or read the following article that was cut and pasted from the
USA article was found at:
http://www.usatoday.com/weather/wdensity.htm
Humidity and air density
Most people who haven't studied physics or chemistry
find it hard to believe
that humid air is lighter, or less dense, than dry
air. How can the air become
lighter if we add water vapor to it?
Scientists have known this for a long time. The
first was Isaac Newton, who
stated that humid air is less dense than dry air in
1717 in his book, Optics.
But, other scientists didn't generally understand
this until later in that century.
To see why humid air is less dense than dry air, we
need to turn to one of the
laws of nature the Italian physicist Amadeo Avogadro
discovered in the early
1800s. In simple terms, he found that a fixed volume
of gas, say one cubic
meter, at the same temperature and pressure, would
always have the same
number of molecules no matter what gas is in the
container. Most beginning
chemistry books explain how this works.
Imagine a cubic foot of perfectly dry air. It
contains about 78% nitrogen
molecules, which each have an atomic weight of 28.
Another 21% of the air is
oxygen, with each molecule having an atomic weight
of 32. The final one
percent is a mixture of other gases, which we won't
worry about. Molecules
are free to move in and out of our cubic foot of
air. What Avogadro
discovered leads us to conclude that if we added
water vapor molecules to
our cubic foot of air, some of the nitrogen and
oxygen molecules would leave
— remember, the total number of molecules in our
cubic foot of air stays the
same. The water molecules that replace nitrogen or
oxygen have an atomic
weight of 18. This is lighter than both nitrogen and
oxygen. In other words,
replacing nitrogen and oxygen with water vapor
decreases the weight of the
air in the cubic foot; that is, it's density
decreases.
Wait a minute, you might say, "I know water's
heavier than air." True, liquid
water is heavier, or more dense, than air. But, the
water that makes the air
humid isn't liquid. It's water vapor, which is a gas
that is lighter than nitrogen
or oxygen.
Compared to the differences made by temperature and
air pressure, humidity
has a small effect on the air's density. But, humid
air is lighter than dry air at
the same temperature and pressure.