wmo-im
1-01-01 Add Water vapour amount fraction, then verify use of wtaer vapour mixing ratio in OSCAR/Requirements
Introduction
The history of this proposal is in #550. The original request was to add Water vapour mixing ratio to WMDS.
The OSCAR/Requirements definition is
3D field of water vapour mixing ratios in the UTLS. Mixing ratio is the mole fraction of a substance in dry air (UTLS, MUS)
Amendment details
10 November 2025 The current proposal is to addd a new code to WMDS:
| notation | name | description |
|---|---|---|
| 12330 | Water vapour amount fraction | The ratio of the amount of water vapour and the total amount of air including water vapour in an air parcel. NB: Concept is related to 'water vapour mass fraction'. For stable gaseous trace constituents in air, the amount fraction of X normally refers to the ratio of the amount of X and the amount of all gases in a dry air parcel, and is called 'dry air amount fraction'. |
Requestor(s)
GCOS, OSCAR/Requirements, @kpremec
Stakeholder(s)
No response
Applications or Systems
- [x] OSCAR/Surface
- [x] OSCAR/requirements
- [ ] GAWSIS
- [ ] Radar/DB
- [ ] OceanOPS
- [ ] WHOS
- [ ] WDQMS
- [ ] GBON Compliance Monitor
- [ ] Other
Expected impact of change
MEDIUM
Consultations
No response
Data exchange
No response
Comments
No response
The latest comment from @meulenvd suggests a different definition instead.
The quotient of mass of water vapour and mass of dry gas. Older literature may have used the term 'water vapour mixing ratio'.
Refer to #550: In fact the statement in OSCAR/requirements for the Variable: Water Vapour Mixing Ratio in https://space.oscar.wmo.int/variables/view/water_vapour_mixing_ratio is wrong, not the definition used in general for [253] (water vapour) mixing ratio. OSCAR/requirements defines "Water Vapour Mixing Ratio", but if fact it deals with another quantity expressed in "the mole fraction of a substance in dry air". That means that the name of the variable (Water Vapour Mixing Ratio) should be replaced by Mole Fraction of Water Vapour (of a sample of moist air) in line with WMO-No. 188, table 4.3 ([4] Mole fraction of the water vapour of a sample of moist air).
@meulenvd I think we have agreed to avoid "mole fraction" and to use "amount fraction", "mass fraction" etc., instead.
@meulenvd I think we have agreed to avoid "mole fraction" and to use "amount fraction", "mass fraction" etc., instead.
I think there are in fact two issues
- The requirement in OSCAR/req is stated for the area 2.5 Atmospheric Climate Monitoring (only) and named the variable incorrectly. I presume Mole Fraction of Water Vapour should be the name, also because the unit is expressed as ppmv. PoC is Caterina Tassone (@tassone ?) and hopefully she will suggest the best solution. Note that I also dislike "mole fraction", but the climate community will have to decide. Chosing for "fraction amount", without indating the dimension (mass, pressure, volume, density) will confuse, I prefer the dimension of the quantity in its name.
- (Water Vapour) Mixing Ratio is already well defined internationally, based on mass ratio (not mass fraction, because it deals with two separate components; fraction is w/r the whole or total).
So I think the current definition in OSCAR/req should be modified: (1) add new variable with mole fraction (if Caterina agrees), and (2) change the definition of water vapour mixing ratio into a correct defintion (as discussed already)
@meulenvd I think we have agreed to avoid "mole fraction" and to use "amount fraction", "mass fraction" etc., instead.
Indeed, "Mole fraction" is to be avoided (according to metrological guidance, because it includes a unit in its name), in favor of "amount fraction".
Please see my comments above
- Mole Fraction of Water Vapour should be used in the OSCAR/requirements were Mixing Ratio is mentioned as variable [so the name of the variable has to be changed] 2 Fraction is to be used when we have a quotient of a single component (eg water vapour) versus the total (all components, so water vapour plus dry air). This is the case for the variable Specific Humidity. 3 Ratio is to be used when we have a quotient of two components or one component versus the rest, like with (water vapour) mixing ratio (water vapour versus dry air)
@meulenvd Please review. From my point of view, this is ready for branch and conclusion.
To my opinion there are 2 items left for discussion:
- It is not clear what the dimension (in ISQ terms, defined in the SI brochure) of amount will be: mass, volume, number of molecules, pressure?
- Fraction shall be used for a single component (humidity) versus the total sum (humidity plus the rest of the air mass/volume/pressure, so all together). In this case with water vapour fraction, the variable is already defined as specific humidity. Ratio is used for e.g. the humidity component versus the rest of the air mass/pressure/volume (not the total). For humidity/water vapour related variables one should choose for specific humidity as primary variable, the rest can be derived from this one using emperical based formulas.
@meulenvd "Amount" according to SI has the unit of mole (my understanding at least). It can also be expressed as a countable, such as atoms or molecules, but it is not the same as mass or volume. To be consistent, I have now removed the word "dry" from the description. and have added another note.
Remove "dry" is fine, so we get the total amount of air in the air parcel (because we say fraction not ratio). That means: "For other gaseous constituents in air, the amount fraction of X normally refers to the ratio of the amount of X and the amount of all gases in a dry air parcel, and is traditionally called 'dry air mole fraction'." will be modified into "For other gaseous constituents in air, the amount fraction of X normally refers to the ratio of the amount of X and the total amount of all gases in an air parcel." ?
@meulenvd Actually, the note I added ("For stable gaseous trace constituents in air, the amount fraction of X normally refers to the ratio of the amount of X and the amount of all gases in a dry air parcel, and is called 'dry air amount fraction'.") is correct, but the "odd one out". The greenhouse gas community always refers to "dry air" to get rid of the variability due to moisture in the air. So, I cannot remove "dry". For ozone, the amount fractions refer to moist air, so are equivalent in concept to the water vapour amount fraction. The question is perhaps if this note should be removed altogether.
https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.11.20 notes:
Team agrees to definition without the extra note. The definition in the OSCAR/Requirements is not correct. @kpremec @reginayasmin to follow up as needed in O/R.
