How to prevent Melasma triggered by heat when planning a beach vacation?

My Melasma darkens horribly every time I have been on a beach vacation even when I use Skin Ceuticals CE Ferulic, under Zinc Oxide sunscreen, under makeup with sunscreen all religiously reapplied every 2 hours -huge sunglasses, and brimmed hat. The only thing I can think that is triggering it is simply the heat itself. I am not on birth control. Can anyone recommend something I can do to prevent the Melasma from being triggered? Will Obagi Blender mixed with Tret prevent it?