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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Received on Fri Nov 21 2008 - 02:20:48 GMT
