What 33girl said is so so true. My hair will frizz out with the slightest bit of humidity no matter how many anti-frizz products I put in it or how many times I straighten the same strand of hair. I usually "scrunch" my hair on rainy/hot days. (If you don't know what this is, you basically get out of the shower with wet hair - not soaking, but wet enough - and put in some moose/curl gel in and basically curl with your hands. It works well if you have even the slightest big of curl in your hair, just as long as your hair isn't naturally pin-straight). I'd reccomend scrunching your hair on the rainy days!
Good luck!
ETA: I didn't really do a good job describing how to scrunch your hair because it's rather hard to explain over the Internet, but
this is a pretty good step-by-step guide. The way I do it is closest to the 2nd method, but try it out before hand and see what works best for you.