[This post is marked as unsolved] What is the calculation for absoulute humidity?
Hello. I'm sorry, I doesn't really understand you question. Could you explain me what you're looking for?
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
Because it includes the computation of vapor pressure. In Weather Station, the 2 computations are done separately.
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)...
I doesn't understand, you want to modify the formula of absolute humidity?
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 = 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.
I like the cold weather. It means you get work done.