# Community Support

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

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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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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/

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

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

Or something completely different?

Thx

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...

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)...

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

Hello!

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

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

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...

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 = m_{W}/m

with m_{W} 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

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