A hygrometer is the instrument used to measure atmospheric humidity — specifically relative humidity (RH), dew point temperature, or absolute humidity. Accurate humidity measurement is critical for weather forecasting, agriculture, HVAC systems, museums, and many industrial processes where moisture content must be controlled.
The classic psychrometer (wet-bulb/dry-bulb thermometer pair) remains a reference standard: one thermometer is wrapped in a wet wick. Evaporative cooling lowers the wet-bulb temperature — the greater the difference from the dry bulb, the lower the humidity. Relative humidity is calculated from the two readings using psychrometric tables or equations. Other types include capacitive sensors (thin polymer film changes capacitance as it absorbs moisture — the most common electronic type), resistive sensors, chilled-mirror dew-point hygrometers (the most accurate, used as calibration references), and the hair hygrometer (human or synthetic hair stretches with humidity — the traditional wall instrument).
Modern automated weather stations use capacitive hygrometers that report RH every minute with an accuracy of ±2–3 %. For scientific applications requiring higher precision (±0.1 °C dew point), chilled-mirror instruments are used but cost ten times more. Radiosondes carry miniature capacitive sensors aloft to profile humidity from the surface to the stratosphere — data essential for numerical weather prediction models. A common source of error is sensor contamination by salt, dust, or pollutants, which is why regular calibration and replacement are necessary.