Martin,
So, it would seem like the potential solutions to the problem you perceive
are
1. Not use the standard name "fraction" in variable names to accommodate
people who are confused when the values are given in percent; or
2. Use the standard name "fraction" and expect people to learn.
I favor #2 because it promotes a common language amongst disciplines.
Regards,
Steve Emmerson
On Wed, Jan 30, 2019 at 3:40 PM Martin Juckes - UKRI STFC <
martin.juckes at stfc.ac.uk> wrote:
> Hi Steve,
>
>
> The issue is more that CF allows more freedom in the choice of units than
> many people expect from a "fraction".
>
>
> A second problem, I think the problem is that I didn't explain the issue
> clearly. In the CMIP data request we are specifying that variables with
> standard name "area_fraction" should be given as percentages. This is
> allowed by the CF convention: an "area_fraction" can be 0.5 or 50%. The
> reason that percentages are being used is because "area_fraction" is being
> used like the proportion of land covered in grass, and people are used to
> having these as percentages rather than fractions. It is all perfectly
> correct as far as the convention goes, but people often interpret the use
> of "area_fraction" for a percentage as an error.
>
>
> Given that we have the framework of allowing flexibility in the choice of
> units, I feel it would be better to avoid having the term "fraction" in the
> standard name, given that it is often interpreted as implying a specific
> choice for the units.
>
>
> regards,
>
> Martin
>
>
> ________________________________
> From: Steven Emmerson <emmerson at ucar.edu>
> Sent: 30 January 2019 21:37
> To: Juckes, Martin (STFC,RAL,RALSP)
> Cc: CF-metadata (cf-metadata at cgd.ucar.edu)
> Subject: Re: [CF-metadata] Putting the units in a CF standard name:
> area_fraction
>
> On Wed, Jan 30, 2019 at 12:54 PM Martin Juckes - UKRI STFC <
> martin.juckes at stfc.ac.uk<mailto:martin.juckes at stfc.ac.uk>> wrote:
>
> I'm afraid I don't understand your comment. When I search for "fraction"
> in the NIST document I find it defined as being a ratio, which is
> inconsistent with the current CF usage. The CF standard name concept
> "area_fraction" is not what NIST or others understand as a "fraction". I'm
> suggesting a change to remove this inconsistency.
>
> Unless we're talking past one another, I'll have to disagree. The NIST
> unit for "mass fraction" is "1" -- even though it's a ratio. A fraction can
> be represented many ways. "1:2", "1/2", and "0.5" all represent the same
> fraction, for example.
>
> Does the CF convention require a particular representation for a fraction?
>
> Regards,
> Steve Emmerson
>
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