wmo-im
water vapour related proposals (1-01-01)
Initial request
OSCAR/Requirements updated Water Vapour mixing ratio in summer of 2024. This issue is an attempt to align with WIGOS metadata codes.
Add water vapour mixing ratio to atmospheric variables (1-01-01)
The term 'water vapour mixing ratio' refers to a mole fraction in OSCAR/Requirements.
| Name | Description |
|---|---|
| Water Vapour Mixing Ratio | "3D field of water vapour mixing ratios in the UTLS. Mixing ratio is the mole fraction of a substance in dry air (UTLS, MUS)" |
The following are relevant in the context of this issue (superseded codes are not mentioned here). Definitions were added in italic where none exists as part of proposed improvements. Further, to align with consistent metrological nomenclature, 'mixing ratio' will be changed to 'fraction'. To help users more familiar with older terminology, additional notes will be added to the descriptions. The existing definition in OSCAR/Requirements is discouraged for the following reasons:
- Reference is made to the vertical layer, UTLS. This is not useful, as the term can be applied to all vertical layers in principle.
- The term 'mole fraction' is strongly discouraged by SI, because there is no concept of the fraction of a unit. Instead, 'amount fraction' should be used.
| Notation | Name | Description | Status |
|---|---|---|---|
| 252 | Integrated water vapour | Total amount of water vapour in a vertical column of air per unit area. Integrated water vapour (IWV) is also known as Precipitable Water Vapour (PWV). | stable |
| 255 | Water vapour pressure | Partial pressure of water in the atmosphere | stable |
| 12249 | Relative humidity (with respect to water) | The ratio of the amount fraction of the water vapour in the air to the corresponding amount fraction if the air were saturated with respect to water at a particular pressure and temperature. Relative humidity is typically reported in percent (%). [Based on definition of R1490 in International Vocabulary of Meteorology (WMO-No. 182). 1992.] | stable |
| 12250 | Specific humidity | The ratio of the mass of water vapour in the air to the mass of a moist air parcel, where the mass of the moist air parcel is the sum of the mass of water vapour and the dry air. Synonym: Moisture content. [Based on definition of M1750 in International Vocabulary of Meteorology (WMO-No. 182). 1992.] | stable |
| 253 | Mass mixing ratio | stable |
The term 253 (Mass mixing ratio) is not defined, but it appears in OSCAR/Surface under >Atmosphere>Humidity. It has been used by (only) 3 stations so far. It is the closest to 'water vapour mixing ratio' of the available terms, however, the latter refers to an amount fraction. The term 253 is a misnomer for 'mixing ratio' used in BUFR and GRIB. In GRIB, it is described as 'mass fraction in air'. Considering its current path, and the ambiguity of the label, and the lack of a description, the term 253 should be better defined.
Amendment details
- see sub-issues
Requestor(s)
GCOS, OSCAR/Requirements, @kpremec
Stakeholder(s)
Enter list of stakeholder(s).
Applications or Systems
- [X] OSCAR/Surface
- [X] OSCAR/requirements
- [ ] Radar/DB
- [ ] OceanOPS
- [ ] WHOS
- [ ] WDQMS
- [ ] GBON Compliance Monitor
- [ ] Other
Expected impact of change
None
Collaborators
No response
References
https://space.oscar.wmo.int/variables/view/water_vapour_mixing_ratio
Comments
This may be related to Water vapour (no definition) which doesn't have a definition.
Publication(s)
Example: Manual on Codes (WMO-No. 306), Volume I.3, WMO Codes Registry, Code table 1-01-01
Validation
No response
related codes:
| notation | path | domains | tags | name | description |
|---|---|---|---|---|---|
| 46 | 252 | \Atmosphere\Humidity | Integrated water vapour | ||
| 49 | 255 | \Atmosphere\Humidity | Water vapour pressure | ||
| 579 | 12165 | \Atmosphere\Humidity | \Atmosphere\Humidity\Water vapour | Water vapour | |
| 661 | 12249 | \Atmosphere\Humidity | atmosphere,humidity | Relative humidity (with respect to water) | |
| 662 | 12250 | \Atmosphere\Humidity | atmosphere,humidity | Specific humidity |
@kpremec proposes to remove this part of the definition from O/R and the new WIGOS codes. "Mixing ratio is the mole fraction of a substance in dry air (UTLS, MUS)"
Proposal is to extract the 2nd sentence "Mixing ratio is the mole fraction of a substance in dry air (UTLS, MUS)" and add it as a note in OSCAR/Requirements.
https://codes.wmo.int/ui/text-search?query=water+vapour produces a long list of entries from grib2, bufr4, and wmdr with clear redundancies. https://codes.wmo.int/ui/text-search?query=humidity produces an even longer list, again with many redundancies. https://codes.wmo.int/ui/text-search?query=watervapor produces one more hit, of a deprecated variable. https://codes.wmo.int/ui/text-search?query=mixing+ratio produces another long list, again with many reduncancies.
Cleaning up these codes would require a collaboration between various teams of WMO and presumably, a more fundamental change to grib2 and bufr4 code list management.
For TT-WIGOSMD, the following are relevant in the context of this issue. Definitions were added in italic where none exists.
| Notation | Name | Description | Status |
|---|---|---|---|
| 252 | Integrated water vapour | Total amount of water vapour in a vertical column of air per unit area. Integrated water vapour (IWV) is also known as Precipitable Water Vapour (PWV). | stable |
| 255 | Water vapour pressure | Partial pressure of water in the atmosphere | stable |
| 12165 | Water vapour | Water in the gaseous phase. [International Vocabulary of Meteorology (WMO-No. 182). 1992.] | stable |
| 251 | Humidity (at specified distance from reference surface) | superseded | |
| 12249 | Relative humidity (with respect to water) | The ratio of the amount fraction of the water vapour in the air to the corresponding amount fraction if the air were saturated with respect to water at a particular pressure and temperature. Relative humidity is typically reported in percent (%). [Based on definition of R1490 in International Vocabulary of Meteorology (WMO-No. 182). 1992.] | stable |
| 12250 | Specific humidity | The ratio of the mass of water vapour in the air to the mass of a moist air parcel, where the mass of the moist air parcel is the sum of the mass of water vapour and the dry air. Synonym: Moisture content. [Based on definition of M1750 in International Vocabulary of Meteorology (WMO-No. 182). 1992.] | stable |
| 256 | Watervapor profile | superseded | |
| 253 | Mass mixing ratio | Ratio of the mass of water vapour to the mass of dry air. Based on AMS Glossary od Meteorology. | stable |
The term 253 (Mass mixing ratio) is the closest match to the term water vapour mixing ratio used in OSCAR/Requirements. Because 'mass mixing ratio' is conventionally used for 'water vapour mixing ratio', but is ambiguous, it is recommended to rename the current WMDR term and include a mention in the definition, as follows:
| Notation | Current Name | New Name | Description | Status |
|---|---|---|---|---|
| 253 | Mass mixing ratio | Water vapour mixing ratio | Ratio of the mass of water vapour to the mass of dry air. Often simply called 'mixing ratio'. Based on AMS Glossary of Meteorology. |
https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.02.20 notes:
@wmo-im/tt-wigosmd to review; Jitze noted that it would be good to simplify this list, and this problem also exists in OSCAR/Requirements
Thanks Anna for this issue.I will look into it.
Best regards,
Anne.
On Thu, Feb 20, 2025 at 3:56 PM Anna Milan @.***> wrote:
https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.02.20 notes:
@wmo-im/tt-wigosmd https://github.com/orgs/wmo-im/teams/tt-wigosmd to review
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https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.02.20 notes:
@wmo-im/tt-wigosmd https://github.com/orgs/wmo-im/teams/tt-wigosmd to review
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In the definition of WVMR, , I suggest deletion of the following "Often simply called 'mass mixing ratio'. Based on AMS Glossary of Meteorology." This part is not true as "mass mixing ratio" can be associated with different variables, for example "Aerosol mass mixing ratio". Even more, the Glossary does not recognize "mass mixing ratio", but only "mixing ratio".
In the definition of WVMR, , I suggest deletion of the following "Often simply called 'mass mixing ratio'. Based on AMS Glossary of Meteorology." This part is not true as "mass mixing ratio" can be associated with different variables, for example "Aerosol mass mixing ratio". Even more, the Glossary does not recognize "mass mixing ratio", but only "mixing ratio".
agree. The term "mixing ratio" has also been widely used in meteorology textbooks, at least in the older versions.
In the definition of WVMR, , I suggest deletion of the following "Often simply called 'mass mixing ratio'. Based on AMS Glossary of Meteorology." This part is not true as "mass mixing ratio" can be associated with different variables, for example "Aerosol mass mixing ratio". Even more, the Glossary does not recognize "mass mixing ratio", but only "mixing ratio".
agree. The term "mixing ratio" has also been widely used in meteorology textbooks, at least in the older versions.
Thanks. First of all, I corrected the links in my original comment. Second, I concede that the AMS glossary refers to 'mixing ratio' without the specification 'mass ...', which makes it all the more ambiguous. Third, I would actually prefer to retire 253 altogether for this reason ... or change to "water vapour mixing ratio"?. @kpremec, @gaochen-larc Any thoughts?
In the definition of WVMR, , I suggest deletion of the following "Often simply called 'mass mixing ratio'. Based on AMS Glossary of Meteorology." This part is not true as "mass mixing ratio" can be associated with different variables, for example "Aerosol mass mixing ratio". Even more, the Glossary does not recognize "mass mixing ratio", but only "mixing ratio".
agree. The term "mixing ratio" has also been widely used in meteorology textbooks, at least in the older versions.
Thanks. First of all, I corrected the links in my original comment. Second, I concede that the AMS glossary refers to 'mixing ratio' without the specification 'mass ...', which makes it all the more ambiguous. Third, I would actually prefer to retire 253 altogether for this reason ... or change to "water vapour mixing ratio"?. @kpremec, @gaochen-larc Any thoughts?
I know this vague... But this is for a historical reason as this term was used before realizing tropospheric trace gases and aerosols. For novice or non-meteorology major researchers, I think this may be a problem...
Another 5 cents for this discussion. When defining the ACTRIS vocabulary, we consulted metrologists (measurement experts) how to name these properties from their point of view. For example, "mole fraction" or "mole mixing ratio" was a complete no-go for them. "Mole" is a unit, and therefore must be part of a property name, just like "voltage" isn't a proper name for "electric tension". We settled on "amount fraction" for what is often called "mole fraction", and on "mass fraction" for what is often called "mass mixing ratio".
I fully support 'amount fraction' and 'mass fraction'. The original proposal has been further developed and a solution is proposed.
..."mole fraction" or "mole mixing ratio" was a complete no-go for them. "Mole" is a unit, and therefore must be part of a property name, just like "voltage" isn't a proper name for "electric tension". We settled on "amount fraction" for what is often called "mole fraction", and on "mass fraction" for what is often called "mass mixing ratio".
@markusfiebig Thank you for the comment. Can you provide links to where "amount fraction" and "mass fraction" are identified/defined?
Amount fraction is largely used in the "Comparison of gravimetric and dynamic dilution of the generation of reference gas mixtures of CO2 at atmospheric amount fraction" https:// doi.org/10.1016/j.measen.2023.100937
Mass fraction is defined -https://1library.net/document/ydrev01z-stoichiometry-and-process-calculations.html
Issue consolidated, branch created. @amilan17 @markusfiebig @gaochen-larc @fstuerzl @kpremec @meulenvd Please review and confirm agreement.
https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.06.12 notes:
waiting for feedback, this is related to #594
Hi Anna. I will look into it and comment.
Kind regards,
Anne Kamau.
On Thu, Jun 12, 2025 at 3:25 PM Anna Milan @.***> wrote:
amilan17 left a comment (wmo-im/wmds#550) https://github.com/wmo-im/wmds/issues/550#issuecomment-2966495179
https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.06.12 notes:
waiting for feedback, this is related to #594 https://github.com/wmo-im/wmds/issues/594
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meeting notes: remove the update description for Water Vapour in this proposal
Hello Anna, Sorry for the quietness on my side,I'm not familiar with ice and snow on this side of the globe. Kindly let me know what I should do next. Enjoy your holidays.
Kind regards,
Anne.
On Tue, Jul 1, 2025 at 4:34 PM Anna Milan @.***> wrote:
amilan17 left a comment (wmo-im/wmds#550) https://github.com/wmo-im/wmds/issues/550#issuecomment-3024052607
meeting notes: remove the update description for Water Vapour in this proposal
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12165 (Water vapour) is dealt with in #594 (decision to retire altogether).
https://space.oscar.wmo.int/variables/view/water_vapour_mixing_ratio is defined as "3D field of water vapour mixing ratios in the UTLS. Mixing ratio is the mole fraction of a substance in dry air (UTLS, MUS)". The definition of what mixing ratio is refered to is clear. The clause "3D field of" goes beyond what the name suggests, and I have questioned this practice of O/R repeatedly. Based on the second sentence in the description, the name should be changed to water_vapour_amount_fraction to disambiguate from the other possible interpretation, namely water_vapour_mass_fraction (=Specific humidity). The terminology 'mixing ratio' should be avoided altogether. This should be addressed in a separate issue.
Thanks for the feedback!
Best wishes,
Anne Kamau. O/S -NFP-Kenya.
On Mon, Jul 28, 2025 at 2:59 PM Jörg Klausen @.***> wrote:
joergklausen left a comment (wmo-im/wmds#550) https://github.com/wmo-im/wmds/issues/550#issuecomment-3126898384
https://space.oscar.wmo.int/variables/view/water_vapour_mixing_ratio is defined as "3D field of water vapour mixing ratios in the UTLS. Mixing ratio is the mole fraction of a substance in dry air (UTLS, MUS)". The definition of what mixing ratio is refered to is clear. The clause "3D field of" goes beyond what the name suggests, and I have questioned this practice of O/R repeatedly. Based on the second sentence in the description, the name should be changed to water_vapour_amount_fraction to disambiguate from the other possible interpretation, namely water_vapour_mass_fraction (=Specific humidity https://codes.wmo.int/wmdr/ObservedVariableAtmosphere/12250). The terminology 'mixing ratio' should be avoided altogether. This should be addressed in a separate issue.
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There appears to be an error in the current code list, namely 253, Mass mixing ratio, no definition This should be corrected to "Mixing ratio" with the following definition. 253, mixing ratio, In meteorology, the ratio between the mass of water vapour and the mass of dry air This is a technical/editorial correction. Cf. #550.
The initial issue comment provides an updated summary with all amendment details.
Fixed!
Kind regards,
Anne Kamau
On Thu, Aug 28, 2025 at 7:01 PM Jörg Klausen @.***> wrote:
joergklausen left a comment (wmo-im/wmds#550) https://github.com/wmo-im/wmds/issues/550#issuecomment-3234082027
There appears to be an error in the current code list, namely 253, Mass mixing ratio, no definition This should be corrected to "Mixing ratio" with the following definition. 253, mixing ratio, In meteorology, the ratio between the mass of water vapour and the mass of dry air This is a technical/editorial correction. Cf. #550 https://github.com/wmo-im/wmds/issues/550.
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https://github.com/wmo-im/tt-wigosmd/wiki/Meeting.2025.09.18 notes: @meulenvd @gaochen-larc, please look at the amendment details to ensure they look good. After this, Joerg or Anna can update the branch.
- for Kruno to review afterwards