Well Mat, never wanting to be out done, and always liking a challenge, here you are. But will it work in PicoLog!!!!
C2 = Dry bulb temperature in Deg C
C3 = Wet bulb temperature in Deg C
Assuming a barometric pressure of 1013 mbar
Relative Humidity =
Thanks to Qwest.net for the formula.
If you put this formula into Excel, you can check the answers at: