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Atmospheric Sciences

What instrument measures humidity?


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March 20, 2016 5:12AM

The instrument that measures humidity is called a hygrometer.

A hygrometer is an instrument that measures relative humidity in the air. One common kind of hygrometer is a psychrometer, a device that measures the temperature of a wet bulb and a dry bulb simultaneously. The wet bulb should be cooler (if it's above freezing), because water evaporates from the bulb, taking energy with it. If the air is more humid, the evaporation is slower, so the bulb is closer to the dry bulb's temperature. A comparison of the wet and dry bulb temperature using a Psychrometric Chart indicates the relative humidity of the air.

Hygrometers can be combined with other systems as the sensor to regulate humidity. The resulting instrument may be called a humidistat or hygristor, which function somewhat like a thermostat adding or subtracting moisture, not heat, to the air. Types of Hygrometers include:

  • a dry bulb thermometer and a wet bulb thermometer with a Psychrometric chart
  • A hygrograph which gives a continuous record of the humidity value using human hair, on a graph
  • A hygristor using lithium chloride is used to find out the humidity values at different altitudes. Hygristor's resistance varies with the humidity and this principle is used to find out the humidity.
  • A humidity indicator card (HIC) is a card on which a moisture-sensitive chemical is impregnated such that it will change color as the humidity changes

In term of units; humidity can be expressed by

  • Absolute humidity: total moisture/volume of air
  • Relative humidity: total moisture/saturated moisture
  • Specific humidity: total moisture/weight of dry air

In term of method, Various equipment can be used to measure water content direct or indirectly.

  • Direct measure by cooling the air down to condensation point.
  • Indirect measure by measure air conductivity and related to presence of humidity. Infrared spectrometry is used to measure presence of moisture from IR absorption.

In term of equipment:

One device is known as a hygrometer. A hygrometer is used to measure humidity.

Not to be confused with the hydrometer which measures the specific gravity of liquids.

A Psychrometer is a type of Hygrometer. A psychrometer consists of two thermometers, one which is dry and one which is kept moist with distilled water on a sock or wick.

As an upgrade in technology there is the trace moisture sensor which is a phosphorous pentoxide (P205), a very hydrophilic material, the resistivity of which changes with the humidity, so it is used in electronic devices designed to make use of the property.


Another tool that could be used is a Cyanometer, an instrument for measuring 'blueness', specifically the color intensity of blue sky. It is attributed to Horace-Bénédict de Saussure and Alexander von Humboldt. It consists of squares of paper dyed in graduated shades of blue and arranged in a color circle or square that can be held up and compared to the color of the sky. The blueness of the atmosphere indicates transparency and the amount of water vapor.

hygrometers and psychrometers