You know exactly when the humidity rises. Before you even step outside, you get the message. You have your own built-in hygrometer: Your hair reacts to humidity by transforming itself into an unmanageable mess. You have chemistry to thank for your bad hair days. What you're seeing is the effect of water on the hydrogen bonds in the structural protein alpha keratin.
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The Dickinson College website The Role of Chemistry in History reports that your hair is made from keratin, specifically alpha keratin. This structural protein also composes your fingernails and toenails, along with horses' hooves, cats' claws and bulls' horns.
Keratin's Coiled Structure
Like all proteins, alpha keratin consists of strings of amino acids -- mostly glycine, alanine and cysteine,suggests The Role of Chemistry in History. These strings form coils. Each turn of the coil maintains its relative position, according to lecture notes published by the Rose-Hulman Institute of Technology, by means of hydrogen bonds and disulfide bonds.
The Effect of Water on Hydrogen Bonds
Exploratorium Magazine Online tells us that when water is present, hydrogen bonds break. This allows the coils of alpha keratin to loosen and stretch. As a result, each strand of hair lengthens. This causes the stereotypical "frizz" effect that some individuals experience during humid conditions.
As conditions dry, hydrogen bonds reform and hair contracts. If you braid your hair or roll it in curlers while it's wet, then allow your hair to dry in that shape, the reforming hydrogen bonds will cause it to retain that shape for some time afterward.
This is not the same chemical process involved in "getting a perm," according to course material from Clackamas Community College. Hairdressers perform permanent waves by using a reducing agent to break disulfide bonds, not hydrogen bonds. After reshaping the hair, the stylist then uses an oxidizing agent to reform the disulfide bonds. The effect of the perm lasts until new growth replaces the reshaped hair.
Because of the effect of humidity on keratin, you can use a strand of hair to make a hygrometer, or instrument that measures changes in humidity. If you want to try it, Exploratorium Magazine Online gives instructions on how to do so. However, hair hygrometers aren't just science fair projects. They were the preferred way of measuring humidity for almost 200 years, from the date of their invention in 1783 to their eventual replacement by electrical hygrometers in the 1960s.
A Bad Beak Day
Alpha keratin isn't the only type of keratin that responds to humidity. According to a publication by the Meyers Group, the beta keratin of a toucan's beak demonstrates reduced stiffness and tensile strength in high humidity as compared with dry conditions.