A ___________ is used to measure relative humidity by comparing wet/dry bulb temperatures.

• A. Hygrometer
• B. Sling Psychrometric
• C. Barometer
• D. Thermometer

Answer: (B)

Measuring relative humidity using two temperatures comes from how evaporation responds to air moisture. A sling psychrometer is built for this: it carries two thermometers, one dry and one with a wick kept wet. You swing it through the air so air moves past both bulbs. Evaporation cools the wet bulb, so its temperature drops below the dry-bulb temperature. The bigger the gap between the two readings, the drier the air; the smaller the gap, the more humid the air. Using those two temperatures with a psychrometric chart or a simple calculation lets you determine relative humidity. A hygrometer measures humidity directly without using the wet/dry bulb method; a barometer measures air pressure; a plain thermometer measures temperature.