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Why does humid weather makes my hair go crazy?

Tom Skilling
Dear Tom,
Why does humid weather makes my hair go crazy?
— Millie Fredrick

Dear Millie,

Human hair absorbs moisture, lengthening up to 3 percent in humid weather.

Hairstylists tell us that curly hair becomes frizzy and straight hair goes limp as the humidity increases and the hair lengthens.

Hair consists of keratin, a protein that is wound into a coil held together by hydrogen bonds. Water causes those bonds to break, allowing the coil to stretch and the hair to lengthen. The bonds re-form when the hair is dried. That’s why hair can be styled by wetting, shaping and frying.

Because of its sensitivity to moisture, human hair is used in hygrometers (weather instruments that measure humidity). As the humidity changes, the lengthening or shrinking of the hair strands causes the hygrometer to display the current relative humidity.