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. Don't just curse the weather every time your hair takes on a mind of its own: the chemical makeup 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 neighboring molecules can react to form a strong disulfide bond. These bonds lock keratin molecules together and maintain their composition whether your hair is wet or dry. You have them to thank for much of your hair's durability and strength.

Hydrogen bonds are much more numerous in hair, and a lot more fickle. They form when a positively charged hydrogen molecule gets caught between two electronegative atoms in a strand of hair. Unlike disulfide bonds, the 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. They're also affected by the humidity in the air.

Instead of breaking down the hydrogen bonds in hair, the right level of humidity produces these bonds in greater numbers. More hydrogen molecules provide more opportunities for hydrogen bonds to form with keratin proteins. As more bonds form, proteins start to double back on themselves, resulting in curly or frizzy hair.

These effects are amplified in hair that's especially dry. Dry hair tends to soak up moisture in the air like a sponge, breaking the strand's outer shaft and making hair look frizzy. This is why hair that's been damaged by heat, chemical coloring, or an overuse of products is often more vulnerable to humid weather.

One way to fight the frizz is by moisturizing your hair before leaving the house. Conditioners, natural serums, and all-around gentler treatment can help cut down on dryness. If that doesn't work, we suggest avoiding tropical climates whenever you can.