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WIGOS Metadata Standard: Semantic standard and code tables
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Request for qualifier to be added: Relative Humidity (%) #381

Closed rnagarjun closed 2 years ago

rnagarjun commented 2 years ago

Proposal Summary

Summary and Purpose

We are working with our network operators to upload station metadata into OSCAR/SURFACE. One of the elements that station observes is Relative Humidity (%). Currently, we noticed Humidity(251) is available; However, it does not specify if it's relative or absolute. We would like a qualifier added to the element to indicate both.

Proposal

last update: 31 May 2022

Add codes for "Relative humidity" and "Specific humidity"

NotationNameDescription
12249Relative 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.]
12250Specific humidityThe 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.]

Supersede "Humidity (at specified distance from reference surface)" with new code 12249.

Notation Name Description Supersede by
251 Humidity (at specified distance from reference surface)   Relative humidity (with respect to water) with notation of 12249

original request

Requesting the following qualifier be added to WMO code registry/Element List.

Stakeholder(s)

@thineshsornalingam - OSCAR/Surface Focal Point (Canada)

Consultations

WMO No. 182 (Meteorological Vocabulary), WMO No. 8 (GIMO), IPCC AR5

Expected Impact of Change

HIGH

gaochen-larc commented 2 years ago

Should we also consider relative humidity over ice? specific humidity is another frequently used term: the (dimensionless) ratio of the mass of water vapor to the total mass of the system (AMS definition)

fstuerzl commented 2 years ago
@rnagarjun, I'm not sure I understand the proposal correctly. Do you suggest a name change for this variable notation name definition
251 Humidity (at specified distance from reference surface)

to "Relative Humidity (%)" or the addition of a new variable?

notation name definition
new id Relative Humidity (%) The ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature. .

In the second case, I think, the expected impact is rather high than low! We need to assume, that OSCAR/Surface users have selected "Humidity (at specified distance from reference surface)" both for absolute and relative humidity, so a variable migration will be complicated. Also the WDQMS already refers to variable 251 as "Relative humidity" (see: https://wdqms.wmo.int/nwp/land_surface/six_hour/availability/humidity/all/2022-04-25/18) , so the current use of this variable is inconsistent.

gaochen-larc commented 2 years ago

AMS definition for relative humidity: The ratio of the vapor pressure to the saturation vapor pressure with respect to water. https://glossary.ametsoc.org/wiki/Relative_humidity

rnagarjun commented 2 years ago

@fstuerzl Our request is indeed the second one. We understand the complication involved in addressing this request. However, As you have indicated, the current use of this variable is inconsistent - which makes it difficult for users to differentiate between relative and absolute humidity. This is precisely the concern brought up by our network operators.

Nonetheless, If WDQMS already refers to variable 251 as "Relative humidity", would it make more sense/easier to rename variable 251, along with the definition provided by @gaochen-larc for RH?

JohnEyre commented 2 years ago

AMS definition preferred to the current proposal - "air-water mixture" gives the wrong impression - air normally contains water vapour (dry air does not). Prefer WMO (AngloEnglish) spelling of "vapour".

gaochen-larc commented 2 years ago

agree with @JohnEyre. We should use the proper spelling to be consistent with the other documentation. I would like raise the need to have relative humidity over ice. I do understand it is much less widely used. However, this may be a good opportunity to cover our bases. For the same reason, we may also consider specific humidity and mixing ratio. I am fine if we can consider these terms later. Thanks!

JohnEyre commented 2 years ago

@gaochen-larc . I agree that it is important in the definition to state that it is RH with respect to water. Some observation methods (e.g. frost point hygrometer) use the principle of saturation with respect to ice, and models need to treat the difference very carefully. However, as the svps with respect to water and ice are fixed curves, it is easy to convert from RH wrt water to RH wrt ice.

gaochen-larc commented 2 years ago

agree with @JohnEyre about the conversion. The issue is awareness. If one assumed RH is overwater, but the actual value was calculated for over ice. This would be a problem. RHi can be over 100%, but RHw cannot. I also realize that the RHi is used more frequently in the upper tropospheric studies for cirrus clouds...

meulenvd commented 2 years ago

I prefer like @JohnEyre, the use of vapour, not water. Relevant is that we deal with gaseous H20, not air containing aerosols. The definition adopted in the past by Cg is given in WMO-No. 8, Vol. I, Chap4 (humidity): Relative humidity U : Ratio in per cent of the observed vapour pressure to the saturation vapour pressure with respect to water at the same temperature and pressure; the term “relative humidity” is often abbreviated to RH. Other parameters, associated to humidity are stated here too and also Cg adopted formulas to be used. More over in WMO-No. 180, WMO Meteorolgical Tables, based on the well known Smithonian Steam tables, and base for the formulas used in upper air dynamics, defines humidity variables accordingly. Note that WMO uses dewpoint, also for temperatures far below freezing point (so water). Icepoint is defined, but not recommended to aoid confusion. For that reason 'water' is used in the definition of RH.

amilan17 commented 2 years ago
  1. For consistency with other codes, I agree with using "vapour" (http://codes.wmo.int/ui/text-search?query=vapour vs http://codes.wmo.int/ui/text-search?query=vapor))
  2. Please consider using definitions from the International Glossary for Hydrology (https://library.wmo.int/doc_num.php?explnum_id=8209)
  3. at a minimum, we should add a definition to 251.
joergklausen commented 2 years ago

My 5 cents, attempting to consolidate:

  1. Spelling will be 'vapour'
  2. 'Humidity (at specified distance from reference surface)' vs 'Relative humidity'. I am fairly convinced that the interpretation of WDQMS that these refer to the same quantity are justified. Hence I would support a process of superseding the existing variable 251 with 'Relative humidity'. The alternative is indeed to introduce one or several new variables and to deprecate the exsting 251. Action: TT-WIGOSMD to decide on 2022-05-19
  3. The definition 1204 from the IGH is a bit convoluted, and it uses strange terminology, namely 'gram-molecular weight'. The UN Vocabulary (unterm.un.org) provides another more succinct definition, with a reference to IPPC AR5 (https://unterm.un.org/unterm/display/record/wmo/na?OriginalId=37d3bcc9-e882-4fec-bb75-91eefa951848): "the ratio of actual water vapour pressure to that at saturation with respect to liquid water or ice at the same temperature". The International vocabulary of Meteorology (WMO 182, https://library.wmo.int/doc_num.php?explnum_id=4712) is a bit dated, but provides definitions for both, relative humidity of moist air with respect to ice (R1480) and relative humidity of moist air with respect to water (R1490). These are identical except for the reference to ice or water: "The ratio of the mole fraction of the water vapour in the air to the corresponding mole fraction if the air were saturated [with respect to ice][with respect to water] at a particular pressure and temperature". I would prefer to define the term 'Relative humidity' as "The ratio of the mole fraction of the water vapour in the air to the corresponding mole fraction if the air were saturated at a particular pressure and temperature". Why? If we were to specify water vs ice in the definition, we really need 2 variables. This would be totally useless for our purposes, because most of the observations are made with commerical sensors, and the users don't even know how they are calibrated by the manufacturer. If the reference to water vs ice is really of importance, it can be specified elsewhere. Action : TT-WIGOSMD to decide on 2022-05-19
gaochen-larc commented 2 years ago

Agree with @joergklausen. Also would like to add a point: humidity, e.g., specific humidity, or water vapor mixing ratio (equivalent to partial pressure) can be directly measured these days. RH would need two measurements: water vapour measurement and ambient temperature measurement. The latter is to calculate the saturation vapour pressure or mole fraction.

amilan17 commented 2 years ago

https://github.com/wmo-im/wmds/wiki/2022-05-19-TT-WIGOSMD-22 notes:

meulenvd commented 2 years ago

Relevant to notice I think is that RH is a ratio and is dimensionless. It depends on what dimensions are used to generate the ratio (pressure, mass, volume?). It'sall explaned in WMO-No. 8, Vol. I, Chap4 (humidity), and with definition: Relative humidity U : Ratio in per cent of the observed vapour pressure to the saturation vapour pressure with respect to water at the same temperature and pressure; the term “relative humidity” is often abbreviated to RH. This definition is estblished already for a long time (also w/r ice) and I will not recommend to change it. Other Humidty related variable are in practice (mass mixing ratio, volume mixing ratio) and also dimensionless but usually the original units are provided (mg/kg). It must be noticed that, because RH is a derived variable (in fact a parameter), it must be reported with air temperature to make sense. Note that atmospheric pressure is relevant too (in association with moist air), where the vapour pressure of liquid aerosols apart from the gaseous vapour pressure has impact (see WMO-No. 8, Vol. I, Chap4 (humidity))

amilan17 commented 2 years ago

I don't think this will be ready for FT22-2

joergklausen commented 2 years ago

@amilan17 I disagree, we should be able to conclude this as discussed last time. Based on the various comments, I think, we should also at this time introduce the term 'specific humidity'. So the final proposal would be (optional parts in italic):

supersede: notation name definition

251

add: notation name definition
new id Relative Humidity (with respect to water) The ratio of the mole fraction of the water vapour in the air to the corresponding mole fraction if the air were saturated with respect to water at a particular pressure and temperature. Relative humidity is typically reported in percent (%). International vocabulary of Meteorology (WMO No. 182)
new id Specific Humidity Synonym: Moisture content. 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. International vocabulary of Meteorology (WMO No. 182)
RMaerz commented 2 years ago

@joergklausen : a good approach to refer to WMO No. 182, short and precicely 👍

amilan17 commented 2 years ago

https://github.com/wmo-im/wmds/wiki/2022-05-31-TT-WIGOSMD-23 notes:

amilan17 commented 2 years ago

@joergklausen @gaochen-larc please branch and issue summary are updated. Please review

gaochen-larc commented 2 years ago

The updated version looks great! Thanks!

joergklausen commented 2 years ago

I concur.

amilan17 commented 2 years ago

ready for fT