[CF-metadata] physically equivalent units

[Philip Bentley]

Hi Jonathan,

Many thanks for clarifying this. Do you think it would it be useful to
add a short footnote to this effect to section 3.3, or is the use of the
phrase "physically equivalent (not necessarily identical)" in that
section self-evident? (I'm thinking here of non-scientists who may be
responsible for creating CF-netCDF datasets.)

Regards,
Phil

On Thu, 2008-11-20 at 18:58 +0000, Jonathan Gregory wrote:
> Dear Phil
> 
> Units can be interconverted if they differ only because of purely numerical
> factors. Apart from such factors all units can be translated into a product
> of a set of "base units" raised to various powers. In SI the base units are
> 
> metre 		m 	length
> kilogram 	kg 	mass
> second 		s 	time
> ampere 		A 	electric current
> kelvin 		K 	thermodynamic temperature
> candela 	cd 	luminous intensity
> mole		mol	amount of substance
> 
> Thus, for instance, a newton N is kg m s-2, a joule J=N m=kg m2 s-2 and a
> pascal is Pa=N m-2=kg m-1 s-2. Dimensionally, Pa and J m-3 are equivalent.
> 
> K, degC and degF are all equivalent.
> 
> A practical way to see whether two units are equivalent is to try to use
> udunits to convert between them.
> 
> $ udunits
> You have: degF
> You want: K
>     <K> = <degF>*0.555556 + 255.372
>     <K> = <degF>/1.8 + 255.372
> You have: Pa
> You want: J m-3
>     <J m-3> = <Pa>*1
>     <J m-3> = <Pa>/1
> You have: K   
> You want: kg
> udunits: Units are incompatible
> 
> Cheers
> 
> Jonathan
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