Steve,
Re your comments in NetCDF Digest [Volume 1 Issue 278]
on my proposed new units for the units database file 'udunits.dat'
> Subject: 19950728 Re: Suggested new units for udunits.dat
> From: Steve Emmerson <steve@xxxxxxxxxxxxxxxx>
> To: netcdfgroup@xxxxxxxxxxxxxxxx
> Date: Fri, 28 Jul 1995 13:19:40 MDT
> > ppt S 1.0e-3 # parts per thousand
> > ppm S 1.0e-6 # parts per million
> > ppb S 1.0e-9 # parts per billion
> > ppt S 1.0e-12 # parts per trillion
>
> `ppt' in the above is ambiguous. But I can add `ppm' and `ppb'.
The fact that I made this mistake proves 'ppt' is a bad idea!
My face is red with embarrassment :-(
> > billion P 1.0e9
> > trillion P 1.0e12
>
> Don't the British use `billion' to mean 1.0e12 (i.e. isn't the British
> `billion' the USA's `trillion')?
The old British billion & trillion were more logical -- 1st 2 letters (bi or
tri) represented the power of million, giving 1e12 & 1e18. In Australia the
meanings have clearly changed (over the last 20 years or so) from these to the
USA usage. I seem to remember a news item a year or so ago to the effect that
the British had officially adopted (I can't remember how) the USA billion (& I
guess trillion). Perhaps some UK reader would like to clarify this.
If there is still real ambiguity, then I guess we can live without billion &
trillion in udunits.dat. However 'ppb' is common & should be included.
> I'm wary of adding too many 2-character abbreviations but will if
> pressed. What do you think?
I see your point and agree.
> > month P year/12 # mean calendar month
>
> I'll add this -- even though a month isn't a twelfth of a year. I
> suppose it might be convenient.
I assure you it will be most useful.
> > sidereal_month P 27.321661 day
> > tropical_month P 27.321582 day
>
> I don't have a reference handy and the above aren't 1/12 of the
> corresponding years. Are you certain about the coefficients?
I have checked these in another reference book & it gave exactly the same
values. They relate to the moon, so they are not 1/12 of corr. year.
I had not realised that you defined 'year' as 3.153600e7 second = 365 days:
year P 3.153600e7 second # exact
It would be clearer if it was expressed in days:
year P 365 day # exact
I suggest also including:
leap_year P 366 day # exact
Julian_year P 365.25 day # exact
mean_calendar_year P 365.2425 day # exact
Mean calendar year is based on fact that in every 400 years there are
303 ordinary years of 365 days: 303*365 = 110595 days
97 leap years of 366 days: 97*366 = 35502 days
total = 146097 days
Thus mean calendar year = 146097/400 = 365.2425 days
I guess one could argue about definition of 'year' and 'month', but I think
year = 365 days, month = 1/12 year are reasonable. For example, our climate
modelling output netCDF files have units 'year', 'month' and 'day' & we
commonly want year = 12 months = 365 days (although it is often necessary to
take into account the number of days in each month).
Harvey Davies, Home: +61 3 9772 5199
CSIRO Division of Atmospheric Research, Work: +61 3 9586 7574
Private Bag No. 1, Mordialloc, Fax: +61 3 9586 7600
Victoria 3195, Australia E-mail: hld@xxxxxxxxxxxx
