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.<br>
<br>-Jason<br><br><div class="gmail_quote">On Mon, Feb 1, 2010 at 10:35 AM, mario frangipane <span dir="ltr"><<a href="mailto:frangix@yahoo.it">frangix@yahoo.it</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
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<div><div>Dear Grads Users,</div><div><br></div><div>I do not understand the example on using the cdiff function at page 76 of the Grads manual.</div><div><br></div><div>For example, if (u,v) is the wind, to calculate du/dy I believe that the calculation should be:</div>
<div><br></div><div>define du=cdiff(u,y)</div><div>define dy=cdiff(lat,y)*(3.1416/180)*(6.37e6) (in meters)</div><div>define dudy=du/dy</div><div><br></div><div>Instead, in the Manual, at pag. 76, I find (example 1):</div>
<div><br></div><div>define du=cdiff(u*cos(lat*3.1416/180),y).</div><div><br></div><div>Anyone can help me to understand?</div><div><br><div><br></div></div></div></td></tr></tbody></table><br>
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