Hi guys.
I have some questions concerning humidity (absolute and relative), dew
point, and air density.

Does it make sense for the weather channel to report a dew point below
freezing?
Is this a result of a calculation from measured relative humidity, air
temperature, and barometric pressure? Or can you actually condense
water out of the air below freezing?

Is there a source of standard functions relating relative humidity,
absolute humidity, pressure and temperature?

If I want a more accurate method of measuring absolute humidity, can I
cool a plate and record the temperature when moisture condenses on the
plate and use that as dew point to calculate relative and absolute
humidity? (This is how they determine humidity in the chamber used to
house the Declaration of Independence without opening the chamber.)

Thanks in advance for any help.

On Mon, 26 Nov 2007 20:28:56 -0800 (PST),
, in
wrote:

+ Does it make sense for the weather channel to report a dew point below
+ freezing?

Yes. The value of the measure goes down as temperture drops further,
and becomes somewhat dubious at about -40. This where super-cooled
water vapor may exist, and it "condenses" directly to ice, and is one
source of aircraft icing.

+ If I want a more accurate method of measuring absolute humidity, can I
+ cool a plate and record the temperature when moisture condenses on the
+ plate and use that as dew point to calculate relative and absolute
+ humidity? (This is how they determine humidity in the chamber used to
+ house the Declaration of Independence without opening the chamber.)

That's one of the most accurate ways of measuring dewpoint. If you
know pressure, temperature and dewpoint, you can calculate any
relative or absolute humidity value you desire.

--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

wrote in message
...
Hi guys.
I have some questions concerning humidity (absolute and relative), dew
point, and air density.

Understanding humidity and dew point:

Dewpoint and relative humidity are related.
Short answer
Water is evaporating and condensing all the time in the atmosphere there is a point where
they are equal..

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.

* "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.


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


Does it make sense for the weather channel to report a dew point below
freezing?
Is this a result of a calculation from measured relative humidity, air
temperature, and barometric pressure? Or can you actually condense
water out of the air below freezing?

Even at subfreezing temperatures most hygrometers are sensitive to relative humidity with
respect to water rather than that of ice because of "supersaturation".


Is there a source of standard functions relating relative humidity,
absolute humidity, pressure and temperature?


Yes, try NOAA or the American Meteorological Society websites, they probably have
glossaries or other resources


If I want a more accurate method of measuring absolute humidity, can I
cool a plate and record the temperature when moisture condenses on the
plate and use that as dew point to calculate relative and absolute
humidity? (This is how they determine humidity in the chamber used to
house the Declaration of Independence without opening the chamber.)

Thanks in advance for any help.

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

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?

* "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, 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?

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

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

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

"Bill Habr" wrote in
et:


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

to the OP, google "vapor pressure water calculator"

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

http://antoine.frostburg.edu/chem/se...roperties.html

HTH

--
Bill Asher

"William Asher" wrote in message
...
"Bill Habr" wrote in
et:


wrote in message
..
.
On Nov 28, 9:22 am, "Bill Habr" wrote:
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/se...roperties.html

HTH

--
Bill Asher

On Thu, 13 Dec 2007 14:10:26 -0600,
Bill Habr , in
wrote:

+ 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' ;-).

Um, "saturation" is proper nomenclature.

But yes, for a given temperature and pressure, relative humidity is
indeed given as

RH (%) = V/Vs * 100%

--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

"I R A Darth Aggie" wrote in message
...
On Thu, 13 Dec 2007 14:10:26 -0600,
Bill Habr , in
wrote:

+ 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' ;-).

Um, "saturation" is proper nomenclature.


In the 18th century it was believed that the atmosphere was like a solution, if one is
adding a chemical to a liquid the liquid may become saturated, however air is not a
liquid. Dalton showed that in the early 1800s. Using the word 'saturation' leads to
misconceptions about what is happening in atmosphere. The reason the relative humidity in
the atmosphere doesn't get much above 100% is because of cloud condensation nuclei that
allow the condensation to form cloud droplets which then coalesce to form cloud drops
which.......






But yes, for a given temperature and pressure, relative humidity is
indeed given as

RH (%) = V/Vs * 100%

--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.

On Fri, 14 Dec 2007 14:03:29 -0600,
Bill Habr , in
t wrote:

+ The reason the relative humidity in
+ the atmosphere doesn't get much above 100%

You need to qualify that statment.

There are conditions where supersaturation occurs, and the values are
significantly larger than 100%.

--
Consulting Minister for Consultants, DNRC
I can please only one person per day. Today is not your day. Tomorrow
isn't looking good, either.
I am BOFH. Resistance is futile. Your network will be assimilated.