G'day bobrun,
What would explain that the air became warm and humid all of a sudden?
There's no outside fresh air being blown directly in the flight deck obviously, so would the icing have also affected the packs operation? Could the heat exchangers have taken ice?
I've experienced a marked OAT rise crossing the ITCZ many times, and in clear air occasionally, and sometimes the first indication is a sudden warming and stuffiness in the cabin. I don't believe it is the packs icing up just that they are not capable of the rapid response needed to counter such a quick OAT increase.
Regards,
BH.