The length of a strand of human hair changes with different relative humidities .
Hair becomes longer with higher relative humidities and shorter with lower relative humidities.
On very humid days, your hair actually becomes longer and this extra length causes the frizziness associated with bad hair days. The fact that the length of hair changes with relative humidity is used in an instrument known as the hair hygrometer. This instrument uses strands of human or horse hair with the oils removed attached to levers that magnify a small change in hair length. An ink pen and rotating cylinder, known as a hygrograph, can provide a record of how relative humidity varied throughout the day. The disadvantages of the hair hygrometer and hygrograph are that they are not as accurate as the sling psychrometer and frequent adjustment and calibration are required.