[CF-metadata] change_in

[Jonathan Gregory]

Dear Burkhardt

> is there any news or any decision on the "change_in_" issue?
No-one expressed any view.

Having thought about it a bit more, I think change_in_ is OK if it has the
same kind of uses as tendency_of. They are really the same except change_in
is extensive in time and tendency_of is intensive. They have the same bounds
in time. The units of tendency_of are X s-1 if change_in has units X. We must
be clear that change_in should *not* be used to describe every possible kind of
difference e.g. between locations or climates. It specifically means the change
in a quantity as a function of time. Do you agree?

Hence let us propose
change_in_atmospheric_water_content_due_to_advection (kg m-2)

Best wishes

Jonathan