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On Nov 28, 9:22 am, "Bill Habr" wrote:
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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.