[CF-metadata] mixing ratio

Pamment, JA (Alison) alison.pamment at stfc.ac.uk
Tue Nov 4 03:34:04 MST 2008

Dear Martin, Heinke and Jonathan,

Martin wrote:
> > > 'water_vapor_mixing_ratio'
> > > water vapor mixing ratio of a parcel of moist air is the ratio of
> the
> > > mass of water vapor to the mass of dry air.
> > I think there is agreement that the existing standard name
> humidity_mixing_ratio has the right
> > definition, but that water_vapor_mixing_ratio would be more
> self-explanatory.  Therefore I will
> > add the name water_vapor_mixing_ratio and make humidity_mixing_ratio
> into an alias.
>     Now I am really puzzled! For all chemical constituents we went a
> long way to make the distinction between
> mass_fraction_of_     and mole_fraction_of_   (see my mail to Philip
> this morning)
>     Why shouldn't this be the case for water vapour, which can also be
> seen as a chemical constituent???
> The "mixing_ratio" label doesn't allow to infer the canonical units.
> So, my suggestion would be
> mass_fraction_of_water_vapour_in_air
> and if you insist on humidity, then we would need an alias to
> mass_fraction_of_humidity_in_air

This raises a couple of points.  Firstly, as Jonathan said, 

> I think the mass fraction of water vapor in air would be the ratio of
mass of water vapour
> to mass of moist air. That's not quite the same. Would you agree?

I think Jonathan is correct because, unless stated otherwise, 'air'
would be assumed to be moist.

The name that currently exists in the table for this quantity is
humidity_mixing_ratio, but I think that we are all agreed to use the
term 'water_vapour' rather than 'humidity'.  This would seem to leave us
with two possible choices for this name: either 

 which was agreed between Jonathan and Heinke, or

which would be more consistent with treating water vapour as a chemical
constituent, as well as a physical one. I think that Martin's point that
water vapour can be regarded as a chemical constituent is a very fair
one. 'Mixing ratio' is not a term that is widely used in standard names
so changing this one name to 'mass_fraction' will not make it
inconsistent with any other group of names.  Personally, I could live
with either solution so I am happy to go with the majority view.  

Best wishes,

==> 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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