Find attached the subroutine that calculates dew point temperature. This should be easily implementable in sigmatoP code. It has been found that humid1 routine in sigmatoP calulates specific moisture so for RH, code from CALCRH subroutine in postproc is used.
For dewpoint calculation formula, one can see the following weblink:
http://www.106.us/gb/Meteorology/algorithms.htm
Moet