FWIW I've heard the same opinion from physical oceanographers
(probably the same ones!) that salinity has no units. I.e. that one
says "the salinity is 35", not "the salinity is 35psu". I interpret
this as meaning that the unit is actually part of the definition of
what salinity is (i.e. if it's not in psu then it's not salinity:
http://en.wikipedia.org/wiki/Salinity).
I agree with Jonathan that this seems strange and could cause
confusion, particularly since I think there has been more than one way
of defining the salinity scale and I have certainly seen different
ways of expressing salinity (whether technically correct or not) in
data files in per mil or other units.
I'm certainly not an expert here though. The question of how to
express this given CF's dependency on udunits (which should surely
itself be reviewed - but that's another discussion).
Jon
On Wed, Jun 17, 2009 at 8:18 AM, Lowry, Roy K<rkl at bodc.ac.uk> wrote:
> Hello Jonathan,
>
> The reason I'm resurrecting this discussion is that we came under strong pressure from a group of physical oceanographers to use 'dimensionless' with no scaling factor instead of PSU for salinity. ?I was raising the issue on the list to see how widespread this opinion was.
>
> Cheers, Roy.
>
> -----Original Message-----
> From: cf-metadata-bounces at cgd.ucar.edu [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jonathan Gregory
> Sent: 17 June 2009 08:06
> To: cf-metadata at cgd.ucar.edu
> Subject: [CF-metadata] Salinity units
>
> Dear Roy and Alison
>
> The intention of recording the units as 1e-3 (dimensionless) was to suggest
> a canonical unit of PSU i.e. approximately the same as parts per thousand.
> However, this is unclear and therefore unsatisfactory. We have discussed this
> before, in fact, and I believe we have decided in principle that the unit
> should be indicated as psu. Since that is not a legal unit in the standard
> udunits.dat, in past discussions we have also decided we would publish our
> own CF version of udunits.dat, including psu as a unit (not convertible to
> other units). The CF udunits.dat should also include sverdrup (=1e6 m3 s-1)
> and bel B (hence decibel dB, dimensionless). The latter is already given as
> the canonical unit for some standard names, although it is not a legal udunit.
>
> I'd suggest that the CF udunits.dat, with these changes, should be put on our
> website, mentioned in 3.1 section about units in the CF standard, and used by
> the CF checker.
>
> Best wishes
>
> Jonathan
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--
Dr Jon Blower
Technical Director, Reading e-Science Centre
Environmental Systems Science Centre
University of Reading
Harry Pitt Building, 3 Earley Gate
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Tel: +44 (0)118 378 5213
Fax: +44 (0)118 378 6413
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Received on Wed Jun 17 2009 - 06:21:07 BST
