Humidity is measured by a device called a hygrometer.

What a hygrometer does

A hygrometer detects how much water vapor is present in the air and converts that into a humidity reading, usually as relative humidity (the percentage of moisture in the air compared with the maximum it can hold at that temperature). Modern hygrometers may also show absolute humidity (grams of water per cubic meter of air) or dew‑point temperature , depending on the model.

Common types of humidity‑measuring devices

  • Mechanical hygrometer / hair hygrometer : Uses a strand of human or synthetic hair that stretches or contracts with humidity changes to move a dial.
  • Digital hygrometer : Uses an electronic sensor (often capacitive or resistive) that changes electrical properties as moisture is absorbed, then calculates relative humidity.
  • Psychrometer (sling or aspirated) : Uses two thermometers—one wet‑bulb and one dry‑bulb—to derive humidity from the cooling effect of evaporation.

Other related instruments

  • Humidistat : Acts like a thermostat for humidity, triggering a humidifier or dehumidifier when moisture levels go above or below a set threshold.
  • Dew‑point meter : Measures the temperature at which air becomes saturated, giving a precise humidity value often used in industrial or meteorological settings.

In short, if you’re asking “what measures humidity,” the core answer is: a hygrometer , with several specialized variants depending on whether you’re in a home, lab, or weather‑station setting.