Why humidity makes hair curly?

Hair is made up of hydrogen bonds. These can only be broken or restructured by heat or moisture. If a person with curly hair uses a flat iron they are restructuring the curl pattern (the hydrogen bonds) in effect straightening their hair. Humidity will break down those hydrogen bonds again returning the hair to it's original curl pattern. This is why when a stylist sets a persons hair, they use heat to break down the hydrogen bonds but there is always a bit of cool hair at the end to "set" that new pattern.