Re: Help with understanding humidity and dew point


"William Asher" <gcnp58@xxxxxxxxx> wrote in message
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"Bill Habr" <billhabr@xxxxxxxxxxxxx> wrote in
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<lance.d.bresee@xxxxxxxxx> wrote in message
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On Nov 28, 9:22 am, "Bill Habr" <billh...@xxxxxxxxxxxxx> wrote:
<lance.d.bre...@xxxxxxxxx> wrote in message


Dewpoint:
The temperature to which a given air parcel** must be cooled at
constant pressure in
order
for equalization* to occur.

Humidity:
The ratio of the vapor pressure to the equalization* vapor
pressure at a given
temperature with respect to water, usually expressed as a
percentage.


Ok,
but I have some new questions:

1 - Would the above definition be for ABSOLUTE humidity, or RELATIVE
humidity? And how can we express the other in the above terms?

Relative humidity


2- The current vapor pressure can be measured with a manometer. Are
the equalization pressures known and established for various
temperatures, say in a NIST table, and well known?

There probably is a table, I remember reading that in the 1950s there
was a punch card library with the infromation.


* "equalization" is usually referred to as "saturation" but
"saturation" is a hold
over
from an18th century understanding of science when it was thought
that air was like a liquid solution.


Have I got this right?
The equalization pressure is the pressure in a system when the
pressures from air and from water vapor are equal,
Equalization pressure is the pressure at a given temperature when
evaporation and condensation are equal

and when the system
pressure decreases any beyond this point (say due to cooling) the
water vapor pressure exceeds the system pressure and water condenses?


Condensation and evaporation are happening all the time, equalization
refers to the point at which condensation and evaporation are equal.
If BOTH pressure and temperature change then the result is a new
equalization point. One way to look at it is that relative humidity is
the change in pressure at a constant temperature and dew point is the
change in temperature at a constant pressure. Something I didn't
mention is that we normally don't see the condesation unless there is
something to condense on like the side of a glass or a car or dust in
the atmosphere.



Is this behavior different between a closed system (say a duct) and
an open system (atmosphere/weather)?

No, it is the same.



** Technically - the temperature and pressure are that of the water
vapor.






Holy mother of christ, what the hell are you going on about equalization
pressure? Your definition of the vapor pressure is more than a little
vague thermodynamically since the vapor pressure of water is independent
of whether there is net condensation or evaporation.

The question is about humidity and dew point.

Relative humidity is the ratio of the vapor pressure to the equalization vapor pressure
at a given temperature with respect to water, usually expressed as a percentage. If you
are from the 18th century or before you can replace the word 'equalization' with the word
'saturation' ;-).


to the OP, google "vapor pressure water calculator"

Here's one example of many available on the web:

http://antoine.frostburg.edu/chem/senese/javascript/water-properties.html

HTH

--
Bill Asher


.

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