[CF-metadata] physical vs dimensional units

From: John Caron <caron>
Date: Thu, 14 Apr 2011 16:13:34 -0000

Hi Jonathan:

On 4/14/2011 2:14 AM, Jonathan Gregory wrote:
> Dear John and Steve
>
> I agree with Steve about this:
>
>> On 04/13/2011 02:25 PM, John Caron wrote:
>>> the point im trying to make is that it would be better to understand
>>> that "mol mol-1" (canonical udunit = 1) is not the same as "m3 m-3"
>>> (canonical udunit = 1).
>> In my opinion, the distinction between "mol/mol" and "m3/m3" is better
>> indicated by the name of the physical quantity being displayed rather
>> than by its unit attribute.
> I would say mol mol-1 and m3 m-3 are actually the same unit. But that doesn't
> mean the quantities with these units are the same. Similarly,
> net_downward_longwave_flux_in_air and net_downward_shortwave_flux_in_air
> are not the same quantity, even though they are both in W m-2.

yes, exactly.

A "physical quantities" library seperate from udunits could provide some
assistance in detecting when two quantities can be added with unit
convertsion, eg "add m and feet" but not "add mol/mol and m3/m3".
> The udunits Unix command doesn't accept garbage garbage-1 as a valid unit,
> and nor should the Java version, I think.

Thats good. I think the java library might be my fault, because I asked
Steve to allow "unknown units" as a half step towards a "physical
quantities" library. in retrospect, best to separate the two.

Regards,
John
Received on Thu Apr 14 2011 - 10:13:34 BST