Hygrometers measure the relative humidity in a given area. There are several different kinds of hygrometer, including two types of electronic hygrometer.
Both capacitive and resistive hygrometer sensors rely on the amount of condensation present, but they measure it differently.
Electronic hygrometers use the dew point to measure humidity. This is possible because the dew point in an area is the point at which water vapor condenses.
The higher the relative humidity in the area, the closer the dew point is to the area's current temperature. Cooling a portion of the air past the dew point results in condensation.
Electronic hygrometers, in general, control the temperature of a surface based on electronic feedback and measure the resulting condensation. The hygrometer reads the air temperature and adjusts the surface temperature of a sensor until condensation forms and can be measured.
Capacitive hygrometers measure condensation by running an alternating current between two plates to test capacitance, which is the ability of something to hold an electrical charge. As the presence of water increases, the ability to hold a charge also increases.
Resistive hygrometers measure the electrical resistance in their sensor. The sensor in a resistive hygrometer absorbs water from the air, and the hygrometer measures the resistance in its sensor.