[gradsusr] vorticity potential

[morteza Miri]

hi 

I am a 
student Climatology.
Wanted to 
grads script calculate potential vorticity. With an example for 20 jun 1962,level 
500hp. I need to have the 
potential vorticity data.Where can I get?
I need potential vorticity datafor pressure level . 

but Site data(ncep/ncar)are sigma levels.
   How I can convert variables in sigma to 
pressure
for nc data?
6e +06 or-5e 
+06 What Analysis is the the potential vorticity maps?
Thanks in advanc
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