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Davis Vp2 Wireless- VWS 14 P90 Serial Data logger
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 « on: April 21, 2009, 12:59:25 AM »

if you go in to  like the alpha numeric or gauge or thermometer there's an thing called virtual temp... what is that for?

Josh
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Josh Patten

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Anthony
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 « Reply #1 on: April 21, 2009, 08:30:34 AM »

Here you go.

Virtual temperature
The virtual temperature (Tv) of an air parcel is the temperature that dry air would have if its pressure and density were equal to those of a sample of moist air. [1] The virtual temperature allows the use of the dry-air equation of state for moist air, except with T replaced by Tv.[2] A formula for the virtual temperature Tv is

where T is the ambient temperature, r is the water vapor mixing ratio (mass of water per mass of dry air), and ε is the molar mass of water vapor divided by the molar mass of dry air (for the Earth's atmosphere this value is 0.622.) In this formula the temperature is in kelvins and the mixing ratio is dimensionless (g/g or kg/kg).

A simpler, approximate formula for virtual temperature is

An approximate formula for virtual temperature using the Celsius temperature and dimensional mixing ratio (g/kg) is

[3]

This climatology/meteorology-related article is a stub. You can help Wikipedia by expanding it.

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Thanks,
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 « Reply #2 on: April 21, 2009, 08:36:42 AM »

Quote
This climatology/meteorology-related article is a stub. You can help Wikipedia by expanding it.
Once I figure out what language the article was written in; it certainly does not appear to be in the English that I know
 « Last Edit: April 21, 2009, 08:49:08 AM by tomcj2 » Logged

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sam2004gp
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Weeeeeeeee!!!!

 « Reply #3 on: April 21, 2009, 08:47:22 AM »

So what is Virtual Temp.  in "laymen's" terms?
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 « Reply #4 on: April 21, 2009, 09:17:28 AM »

It's the temperature I'd feel if I were actually at home more often...
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 « Reply #5 on: April 21, 2009, 09:18:32 AM »

I've also wondered some times what viritual temp is, is it some ordinary "term" or as sam2004gp said "So what is Virtual Temp.  in "laymen's" terms?"
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sam2004gp
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Weeeeeeeee!!!!

 « Reply #6 on: April 21, 2009, 10:31:59 AM »

Here is the definition from the most current VWS manual, that I found.
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 « Reply #7 on: April 21, 2009, 10:44:09 AM »

lol thanks i think i kind of got an idea of what it is now.

Josh
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Josh Patten

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ironton
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 « Reply #8 on: April 21, 2009, 10:53:58 AM »

This may be a bit late since Sam's post above.

Just in case this definition is still confusing to some I will try to give my interpretation of what it means.  (but mostly because I had all this typed up)
I will admit to going out on a limb here so correct me if I am off base. (always willing to learn)

We can all relate to the extra-cool feeling when the evening air is damp and we probably have also heard the expression used by folks living in the desert states "....yeah, but it's a dry heat."

Moisture gives air the ability to hold more heat per given volume.  90F air with 90% relative humidity seems hotter than 90F air with 10% humidity.  The same can be said for the cooling effect of 50F air with more or less relative humidity.

The virtual temperature calculation attempts to remove the moisture from the air and tell us what the air temperature would have to be for the "same feel" if it was at standard conditions.

To go a bit further....
The virtual temperature definition says it allows meteorologists to use the "equation of state for dry air".  The equation of state takes into account a few laws of physics - notably Boyle's Law and Charles's Law.  They simply state that (1) a volume of gas will be smaller if the pressure is increased and (2) a volume of gas will be smaller if the temperature is lower (assuming all other terms are the same).

The virtual temperature definition implies that in order to perform any type of calculation on the air, meteorologist must convert the temperature to standard conditions.  In physics this is known as STP (or Standard Temperature and Pressure).

Now, does anyone know why meteorologists might need to perform these calculations?  Perhaps to estimate the energy in a system?

Ok, not sure if I helped or not - but that is my attempt.
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 « Reply #9 on: April 21, 2009, 11:30:07 AM »

"Virtual temperature is a fictitious temperature that takes into account moisture in the air. The formal definition of virtual temperature is the temperature that dry air would have if its pressure and specific volume were equal to those of a given sample of moist air. Virtual temperature allows meteorologists to use the equation of state for dry air even though moisture is present."  Now if I could only figure out the relationship of bouyancy fluctuation and adioabtic cooling...  LOL
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 « Reply #10 on: April 21, 2009, 11:58:51 AM »

. . . . .
Now if I could only figure out the relationship of bouyancy fluctuation and adioabtic cooling...  LOL

Adiabatic cooling --- if a parcel of air moves upward (as it flows over a hill or mountain, for example), it enters a region of lower pressure and consequently expands. Because the parcel expands, its temperature decreases (ideal gas equation of state). Thus, the parcel has cooled, not because energy (heat) was removed from it, but simply because its temp decreased by expansion. That's adiabatic cooling.

Conversely, if a parcel of air moves downward, it enters a region of higher pressure and consequently is compressed. Because the parcel is compressed, its temperature increases (ideal gas equation of state). Thus, the parcel has warmed, not because energy (heat) was added to it, but simply because its temp increased by compression. That's adiabatic heating.

Here's an example: have you ever used a can of compressed air to dust a computer or keyboard? As you spray the compressed air out, the can starts feeling very cold. That's because as gas is released from the can, the compressed gas remaining in the can expands and cools, not because energy (heat) is being removed from it, but because it is expanding, so its temp decreases.

EDIT:  Adiabatic cooling is sometimes called expansion cooling. Adiabatic heating is sometimes called compression heating.
 « Last Edit: April 21, 2009, 12:31:49 PM by WeatherBeacon » Logged

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 « Reply #11 on: April 21, 2009, 03:17:11 PM »

Quote
It's the temperature I'd feel if I were actually at home more often...

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