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 > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
