Hygrometers are used to measure relative humidity and are based on the hygroscopic properties of hair or synthetic fibers, which change their length with changes in humidity.
While natural hair can also be used at temperatures below 0 °C, hygrometers with synthetic fibers react extremely slowly in this temperature range and are therefore not recommended for outdoor use, as the inertia of the synthetic fiber increases with decreasing air temperature.
Hair hygrometers require regular regeneration (every few weeks); this is generally not necessary for hygrometers made of synthetic fibers.
