From AMS Glossary
A hygrometer that measures relative humidity by means of the variation in length of a strand of human hair.
The length variation of a properly treated hair is 2%–2.5% when the humidity changes from 0%–100%. The hair hygrometer is considered to be a satisfactory instrument in situations where extreme and very low humidities are seldom or never found. The rate of response is very dependent on air temperature; the lag time increases with decreasing temperature. For air temperatures between 0° and 30°C and relative humidities between 20% and 80%, a good hair hygrometer should indicate 90% of a sudden change in humidity within about three minutes.