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[UDUNITS #MZJ-180852]: udunits question molecules/cm^2

James,

> Great, guess I'll see about getting the latest version installed here.
> 
> Just one final question. For an atmospheric variable such as NO2 column amount
> expressed in units of molecules/cm^2, if in a netcdf/CF compliant file one
> enters units as cm-2. How will an application necessarily know this refers to
> something like molecules/cm^2? Most applications will probably just look at
> the units and print the units as is.

Because there are many physical quantities that have compatible units but 
totally different semantics (e.g., energy and torque), applications must use 
more than just the unit specification to deduce the semantics of a physical 
quantity. This is why conventions exist on standard variable names.

> Similarly for a volume mixing ratio product, most of my data sets I see
> specify the units as 'vmr'

I don't know the unit "vmr".

> or 'ppm', but neither of those are SI units.

I believe the UDUNITS package correctly handles the unit "ppm".

> The
> udunits requests units = "1", and specify the "vmr" in the long_name. I don't
> know if applications then list a color table with for example a label 'NO2
> column amount (vmr)' and with units '1'. The units '1' would look awful in my
> opinion.

An application can, of course, use the unit specification of a variable in its 
output display. In my opinion, however, good applications will allow the user 
to specify the desired output unit specifications and the application will 
perform the appropriate conversions.

> ------------------------------------------------------------------------------
> | James E. Johnson                      | address:                           |
> | Wyle Information Systems, Inc.        |  NASA Goddard Space Flight Center  |
> | e-mail: address@hidden        |  Distributed Active Archive Center |
> | phone:  301-614-5121                  |  Code 610.2, Bldg 32, Room S130G   |
> | fax:    301-614-5268                  |  Greenbelt, MD  20771              |
> ------------------------------------------------------------------------------

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: MZJ-180852
Department: Support UDUNITS
Priority: Normal
Status: Closed