I live in heat & humidity too, which I despise (whole other story)...
For me, using MMF + tylose + a microwave is the answer to cracking.
I honestly don't even notice the weather when I model something.
My fondant performs the same any time of the year.
(A member at Flickr posted pics of her pieces literally melting from the humidity...she was using gumpaste & fondant mixed...I've never had that happen either).
** great advice, I never would have thought of that until it was to late. just beginning my journey into modeling. Thanks!