Jonathan,
I think you meant to say that if a physical quantity has a different
dimensionality (not unit), then we have to give it a different name.
In my opinion, what's needed in this case is a package that understands
co-ordinate transformations -- in order to convert, for example, values in
units of "1e15/cm2" to values in units of "mol/m2". This is a rather simple
example and Maarten makes a good (though not yet convincing to me) argument
for simply modifying the UDUNITS database. One can imagine, however, more
complicated cases in which simple unit conversions are not possible (e.g.,
converting between altitude and pressure). Such a package would be easily
capable of handling Maarten's conversion.
Regards,
Steve Emmerson
On Wed, Jan 7, 2015 at 8:50 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk>
wrote:
> Dear Maarten
>
> > >A mole is also a udunit, so mol m-3 and m-3 are different units, and
> quantities
> > >with those canonical units also have to have different standard_names.
> The
> > >standard_name indicates whether the quantity refers to number
> concentration or
> > >molar concentration.
> >
> > 1e15 molecules/cm2 NO2 is a valid column amount for NO2, but at the
> > same time I'm trying to convince my colleagues that this quantity
> > should be expressed in mol/m2 (specifically 16.6 micromol/m2 for
> > said column amount). These express the same quantity and should
> > _not_ use different standard names, not in the case of molecules. In
> > the end the number density should become extinct for molecular
> > species, switching completely to mol based units. Having different
> > standard names will only serve to hinder the transition.
>
> Ah, I see (perhaps! :-) Is the point that "mole" means a particular number
> of
> molecules, so there should *not* be a distinction between [molecules] m-3
> and
> mol m-3? Yes, that's a good argument, but in CF we are following SI, and
> mol
> is an SI unit, so a mole of NO2 is dimensionally different from 6e23
> molecules
> of NO2. If it has a different unit, we have to give it a different standard
> name. udunits follows SI as well; you can't convert mol into a plain
> number.
>
> Best wishes and thanks for the explanation
>
> Jonathan
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