Thanks Samer,
I will try selecting another station to see if it resolves the issue.
How do I check that a station is reporting all metrics correctly? If I go to wunderground site, the station (IGAUTENG163) reports 36% humidity.
I would have thought the Zimmerman formula would assume any missing data to be as per the expected norm. I.e in teh absence of a temp reading it would use 21C base temp.
Cheers
Sean