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[This post is marked as unsolved] What is the calculation for absoulute humidity?  

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SetMin
(@setmin)
Trusted Member

What is the calculation for the two selectable absoulute humidity measurements?

Thanks 👍 

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Posted : 09/01/2019 10:37 am
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Hello. I'm sorry, I doesn't really understand you question. Could you explain me what you're looking for?

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Posted : 14/01/2019 8:35 am
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SetMin
(@setmin)
Trusted Member

My netatmo measure relative humidity and temperature.

To create the absolute humidity there must be any kind of calculation in the plugin.

What is the calculcation to display the absolute humidity?

 

Thx

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Posted : 14/01/2019 1:15 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

You want to know the formulae used by Weather Station, is that correct?

If so, here it is for partial absolute humidity:

pAH = 0.622 * pVap / (p - pVap)

where pVap is the partial vapor pressure

and for saturation absolute humidity:

sAH = 0.622 * sVap / (p - sVap)

where sVap is the saturation vapor pressure

Note: you can check all the formulas used in /live-weather-station/includes/traits/CommonUtilities.php

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Posted : 14/01/2019 1:43 pm
SetMin liked
SetMin
(@setmin)
Trusted Member

Thx.

Why is the formula so short compared to the formula used here?
https://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/

 

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Posted : 14/01/2019 2:19 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Because it includes the computation of vapor pressure. In Weather Station, the 2 computations are done separately. 

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Posted : 14/01/2019 2:31 pm
SetMin liked
SetMin
(@setmin)
Trusted Member

Hi @pierre-lannoy

Is the result of the two formulas then the same plus/minus some percent?

Or something completely different?

Thx

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Posted : 15/01/2019 9:58 am
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Hello.

Unfortunately it will be different because its formula is wrong. You can not compute absolute humidity in g/m³ if you don't involve the air density. To compute air density, you must use current atmospheric pressure. The computation you point out doesn't use pressure and, so, can not be in g/m³...

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Posted : 15/01/2019 5:04 pm
SetMin liked
SetMin
(@setmin)
Trusted Member

Hi Piere,

The text on the site says "...requires an added algorithm that generates saturation vapor pressure as a function of temperature".

I understand this like that the pressure IS  in the forumula.

best regards

SetMin

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Posted : 15/01/2019 5:10 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Ah, yes, you're right, I did not see that.  😇 

The vapor pressure is in the formula, not the atmospheric pressure (the pressure measured at the station level)...

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Posted : 15/01/2019 5:18 pm
SetMin liked
SetMin
(@setmin)
Trusted Member

Hi Pierre,

you wrote that all formulas are listet in this file /live-weather-station/includes/traits/CommonUtilities.php

So if i change an existing forumula to my wished "absolute humidity" forumula then i have what i need, or?

Can it be so easy? Or is there something that i miss?

Thanks

SetMin

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Posted : 24/01/2019 11:53 am
Pierre Lannoy
(@pierre-lannoy)
Member Admin

Hello!

I doesn't understand, you want to modify the formula of absolute humidity?

If you like Weather Station, please consider to make a review to help make it known. That would be the best way to thank me...

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Posted : 24/01/2019 11:55 am
SetMin liked
SetMin
(@setmin)
Trusted Member

Yes, I want to modify your formula of absolte humidity to another formula which represents the data in g/m³.
The formula that i want to use is described here: Link

 

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Posted : 24/01/2019 12:00 pm
Pierre Lannoy
(@pierre-lannoy)
Member Admin

OK. You can modify this formula here, but:

  • the result of this formula will be interpreted by Weather Station as a g/kg value (so all subsequent uses will be "wrong")
  • at next plugin update, your modification will be overwritten 

But it's up to you!

Once again, I allow myself to insist: the formula you want to implement is wrong, it can't be in g/m³ because there's no air density involved...

If you like Weather Station, please consider to make a review to help make it known. That would be the best way to thank me...

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Posted : 24/01/2019 12:10 pm
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Chris
(@zerog981)
Reputable Member 🇩🇪🇨🇭Editor

Maybe I can do something to clear things up and provide a more "scientific" approach. I took this information from the textbook Meteorologie by Klose (Sorry it's in German). According to this book the content of vapor in dry air can be measured in two ways:

  • using vapor pressure
  • using the mass of vapor (in kg) contained in a specific volume (e.g. 1 m³), which can be interpreted density. This is also refered to as absolute humidity. So in fact kg/m³ is the unit of absolute humidity.

But...as Pierre already mentioned absolute humidity (i.e. now the density of the vapor) depends on vapor pressure and temperature. Therefore the absolute humidity in kg/m³ is useless when comparing measurements taken at different elevations/temperatures. It's like comparing the mass of two objects without considering their size.

According to Klose a solution to this problem is to introduce specific humidity, which is defined as

s = mW/m

with mW being the mass of water vapor and m being the total mass of wet air. The unit of course is kg/kg (or g/kg).

So I'm with Pierre, because despite of the terms absolute and specific, both describe the same physical property though the latter one is way more useful.

Chris

This post was modified 6 months ago 2 times by Chris

I like the cold weather. It means you get work done.

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Posted : 24/01/2019 7:52 pm

  
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