Curly hair tends to be much drier than straight hair because it is easier for the oils secreted from the scalp to travel down the shaft of a straight hair than a curly one (this is why curly hair often turns into frizzy hair).
And as anyone with curly hair knows, humidity can make your hair even curlier (or frizzier). The reason: Hair fiber absorbs the water and forces the shaft to revert to its original (less straight) structure.
Hair’s chemical structure, it turns out, makes it unusually susceptible to changes in the amount of hydrogen present in the air, which is directly linked to humidity. Most of a hair’s bulk is made up of bundles of long keratin proteins, represented as the middle layer of black dotstightlypacked together in the cross-section at right.
This type of bond is permanent—it’s responsible for the hair’s strength—and isn’t affected by the level of humidity in the air.
But the other type of connection that can form between adjacent keratin proteins, a hydrogen bond,
is much weaker and temporary, with hydrogen bonds breaking and new ones
forming each time your hair gets wet and dries again. (This is the
reason why, if your hair dries in one shape, it tends to remain in
roughly that same shape over time.)