Which two measurements does a sling psychrometer use to estimate relative humidity?

• A. A dry-bulb temperature and a wet-bulb temperature
• B. Relative humidity measured directly with a digital sensor
• C. A dew point and a dry-bulb
• D. A force measurement of air flow

Answer: (A)

The key idea is that relative humidity is inferred from how much cooling occurs when air passes over a wet surface. In a sling psychrometer, you have two thermometers: one reads the ambient air temperature (dry-bulb), and the other has its bulb wrapped in a wick saturated with water (wet-bulb). As air swirls around, water evaporates from the wick, cooling the wet-bulb. The amount of cooling depends on how much moisture the air can still hold—dry air allows more evaporation and a larger drop on the wet-bulb, while humid air limits evaporation and the wet-bulb stays closer to the dry-bulb temperature. By comparing the two temperatures, you can estimate relative humidity using a psychrometric chart or calculation. The other options don’t fit because direct humidity readings, dew point with only a dry-bulb, or air-flow force measurements aren’t how a sling psychrometer operates.