People with long hair know that even the most meticulously primped do is at the mercy of the weather. When the air is humid, hair can start to crimp, curl, or frizz the moment you step outside. The chemical make-up of your locks is also to blame.
A strand of hair is composed of bundles of a fibrous protein called keratin, and the shape and structure of your hair is determined by how these proteins bind together. Keratin molecules contain high amounts of the amino acid cysteine, which in turn contains sulfur atoms. When two keratin chains are nearby, the sulfur atoms from the neighbouring molecules can react to form a strong disulfide bond. These bonds lock keratin molecules together and maintain their composition whether hair is wet or dry.
Hydrogen bonds form when a positively charged hydrogen molecule gets caught between two electronegative atoms in a strand of hair. Hydrogen bonds that form between these atoms are easily dissolved when wet. They break down and form anew each time you take a shower and dry your hair. Instead of breaking down the hydrogen bonds in hair, the right level of humidity produces these bonds in greater numbers. These effects are amplified in hair that's especially dry. Dry hair tends to soak up moisture in the air like a sponge. This is why hair that’s been damaged by heat, chemical coloring, or an overuse of products is often more vulnerable to humid weather.
Source: www.mentalfloss.com
