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 <[email protected]> Sent: 30 January 2019 21:37 To: Juckes, Martin (STFC,RAL,RALSP) Cc: CF-metadata ([email protected]) 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 <[email protected]<mailto:[email protected]>> 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 _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
