use of function cdiff

[Jason Snyder]

the cos(lat*3.14/180),y) is used to convert degrees to meters.  I know that
for x which is longitude you have to account for the fact the length of a
degree longitude changes with latitude and that is accounted for when
calculating du.

-Jason

On Mon, Feb 1, 2010 at 10:35 AM, mario frangipane <frangix at yahoo.it> wrote:

> Dear Grads Users,
>
> I do not understand the example on using the cdiff function at page 76 of
> the Grads manual.
>
> For example, if (u,v) is the wind, to calculate du/dy I believe that the
> calculation should be:
>
> define du=cdiff(u,y)
> define dy=cdiff(lat,y)*(3.1416/180)*(6.37e6)  (in meters)
> define dudy=du/dy
>
> Instead, in the Manual, at pag. 76, I find (example 1):
>
> define du=cdiff(u*cos(lat*3.1416/180),y).
>
> Anyone can help me to understand?
>
>
>
>
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