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: alison.pamment at stfc.ac.uk <==
------
J Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Fax: +44 1235 446314
Rutherford Appleton Laboratory Email: alison.pamment at stfc.ac.uk
Chilton, Didcot, OX11 0QX, U.K.
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Received on Wed Nov 26 2008 - 07:30:13 GMT
