Dear Jonathan, Philip, Martin and Christiane, Jonathan wrote: > > For water vapour, the common terms are (I believe) humidity mixing ratio and > specific humidity. > Specific humidity means mass_fraction_of_water_vapor_in_ambient_air= > (water vapour)/(air including water vapour), while humidity mixing ratio = > (water vapour)/(dry air) Both specific_humidity and humidity_mixing_ratio are > standard names at present and I propose we keep them. For other X we could use > different constructions. > The above names and definitions are indeed those currently used in the standard name table. In both cases the definitions in the table make clear whether the ratio is calculated relative to moist or dry air.
Both humidity_mixing_ratio and specific_humidity are terms I am used to using in meteorology, but the question seems to be whether humidity_mixing_ratio is a confusing name for atmospheric chemists. Martin has indicated that he would prefer a single naming convention for all chemical quantities, Philip has previously said that he prefers mass_fraction_of_water_vapor_in_dry_air and Christiane has queried the definition of "mixing ratio". Heinke, whose proposal began this conversation, was happy with water_vapor_mixing_ratio but did not easily discover the term humidity_mixing_ratio. Judging by the discussion so far I would say that humidity_mixing_ratio is not the ideal choice of name - it could perhaps be regarded as meteorological jargon - so I think we should consider the possible alternatives. Philip asked the question: > > If we do choose humidity_mixing_ratio, would it make sense to create an alias > from mass_fraction_of_water_vapor_in_dry_air, so as to guide chemists to the >'special case'? > Unfortunately, it would not be possible to do this because aliases in CF are used only as a way of recording previous versions of existing standard names, for example, when a name is changed for reasons of clarity or to correct a spelling mistake. It is not intended to be used as a means of creating synonyms between current names or as a navigation aid to the standard name table. If we changed the name to mass_fraction_of_water_vapor_in_dry_air then humidity_mixing_ratio would become its alias, but we couldn't do it the other way round. In an earlier posting Jonathan wrote: > > I agree that your definition is exactly what humidity mixing ratio means. > Here's a more explicit statement of what it means: > mass_ratio_of_water_vapor_to_dry_air_in_air > I'm not sure that Jonathan was actually putting this forward as a proposal but I think it's worthy of serious consideration. It states clearly what is being calculated (mass_ratio) and the two 'species' involved (water_vapor and dry_air). This wording makes clear that the species in the numerator is not also being counted in the denominator and thus gets round the "water vapour in dry air/fat in fat-free yoghurt" problem. In practice, I don't think we would need the 'in_air' because I think it's highly unlikely that we will ever have to contend with names such as mass_ratio_of_water_vapor_to_dry_air_in_sea_water (but I'm prepared to be corrected on that point!) Also, the existing humidity_mixing_ratio name doesn't say that it is 'in_air' - that is just assumed. Thus, I now suggest that we rename humidity_mixing_ratio to mass_ratio_of_water_vapor_to_dry_air. This will have consequences for other names which I describe below. If we adopt mass_ratio_of_water_vapor_to_dry_air as a name then for consistency we should change the existing mass_fraction_of_X_in_air names to also read mass_ratio_of_X_to_Y. This would also give us the opportunity to change Y to be either 'dry_air' or 'ambient_air' as appropriate, or retain it as simply 'air' in cases where we want to be vague or where the mass of water vapour is considered unimportant. Martin's and Christiane's postings in this thread have convinced me that the additional clarity of being able to state dry|ambient air is desirable. Apparently, many chemists would assume dry air while personally it would not have occurred to me to assume anything other than ambient air unless told otherwise. Currently neither the names nor the definitions clarify this point. I hope these suggestions are an acceptable compromise between the needs to: (a) have a consistent chemical naming convention; (b) avoid confusion about which masses/species are included in the numerator and denominator of the calculation. Best wishes, Alison ==> Please note new email address: [EMAIL PROTECTED] <== ------ J Alison Pamment Tel: +44 1235 778065 NCAS/British Atmospheric Data Centre Fax: +44 1235 446314 Rutherford Appleton Laboratory Email: [EMAIL PROTECTED] Chilton, Didcot, OX11 0QX, U.K. -- Scanned by iCritical. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
