Date: Fri, 28 Jun 2002 16:43:33 -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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Sorry for the waste of bandwidth but my last post was so poorly
formatted I had to send it again.
I agree with Arlen completely.
Humid air is less dense than dry air and will consequently be less
effective at cooling.
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 Today 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.