Which instrument is used to measure humidity?

• A. Psychrometer
• B. Barometer
• C. Hygrometer
• D. Thermometer

Answer: (A)

Humidity is the amount of water vapor in the air, and one classic way to determine it is by comparing how quickly water can evaporate from a surface in the air. A psychrometer does exactly that by using two thermometers: a dry-bulb thermometer and a wet-bulb thermometer (the latter has a wick soaked in water). As air passes by, evaporation from the wet wick cools the wet-bulb thermometer. The temperature difference between the dry and wet bulbs depends on how much water vapor is already in the air—the drier the air, the larger the difference; the more humid the air, the smaller the difference. By reading that difference and using a chart or calculation, you can find the relative humidity.

In contrast, a barometer measures air pressure, a thermometer measures temperature, and a hygrometer is a device that directly senses humidity through changes in materials or electronics. The psychrometer is a practical instrument specifically tied to determining humidity via the temperature-difference method, which is why it’s used to measure humidity.