If the air is dry, greater amounts of water will evaporate causing greater cooling of the wet bulb thermometer. The amount of cooling is determined by comparing the wet bulb thermometer measurement to that of a dry bulb thermometer. When there is little evaporation occurring because the air is humid, there will be minimal cooling on a thermometer that is covered with a thin cloth that is wet. Because temperature and wet bulb temperature are known, both relative humidity and dewpoint temperature can be quickly calculated. The hair hygrometer works because hair responds to the relative humidity of the air.
Hair acts as a natural hygrometer.
When the air becomes more humid hair shrinks, resulting in curlier hair. Looking at the design of the instrument, this will cause the arrow pointer to point up. When the air dries hair lengthens and becomes straighter. This causes the pointer to move down. There will be inevitable experimenting with the instrument to see how it works, if it works, and making measurements.