Which instrument uses a wet and dry bulb to determine relative humidity?

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

Answer: (D)

Measuring relative humidity with a wet-bulb and a dry-bulb thermometer relies on how evaporation cools the wet bulb. The dry-bulb thermometer provides the ambient air temperature, while the wet-bulb thermometer has its bulb wrapped in a wet wick. As water evaporates from the wick, it absorbs heat and cools the bulb. The amount of cooling depends on how much moisture is already in the air: in dry air, evaporation is rapid and the wet-bulb temperature drops significantly below the dry-bulb temperature; in humid air, evaporation slows and the two readings are closer together. By comparing these two temperatures—directly or via a psychrometric chart—you determine the actual vapor pressure relative to the saturation value, giving relative humidity. The other instruments don’t use this pair of readings: a thermometer only measures temperature, a barometer measures air pressure, and a hygrometer can measure humidity by other methods.