calculations with "undefined" variables...

[Brian E. Doty]

It has to do with the way GrADS handles constant values
internally.  When you use a constant value (eg, 100)
GrADS treats it like a grid, and creates all the appropriate
internal structures that describe a grid.  It assigns
an undefined value of -9.99e33 for that grid.

When one then does a calculation between two grids,
(eg, divide by 100), where the different grids have
different undefined values, GrADS takes care of that, but
has to decide what the undefined value of the output
grid will be.  Which will be taken from one of the input
grids.   It is hard to predict which undefined
value will rule the day; it depends on the order of
operations.

I'll probably change the way GrADS handles missing data
internally in the next version (GrADSv2)... Brian