The amount of water in the air, as measured with a hygrometer and sometimes expressed as a wet-bulb temperature, is known as which humidity measure?

• A. Absolute humidity
• B. Relative humidity
• C. Vapor pressure
• D. Specific humidity

Answer: (B)

Relative humidity measures how much water vapor is present in the air as a percentage of what the air could hold at the current temperature. A hygrometer that uses a wet-bulb thermometer ties into this by gauging the cooling effect of evaporation. If the air can take up a lot of moisture (dry conditions), evaporation cools the wet bulb more, and the wet-bulb temperature drops significantly; if the air is already moist, evaporation is limited and the wet-bulb temperature stays closer to the dry-bulb temperature. From those readings you estimate the actual vapor pressure relative to the saturation vapor pressure, giving a percent value for relative humidity.

Absolute humidity, vapor pressure, and specific humidity describe different properties: amount of water vapor per volume, the vapor’s partial pressure, and the water vapor mass per mass of moist air, respectively. None are inherently expressed as a percentage based on how full the air is of moisture like relative humidity is, which is why the wet-bulb-based measure points to relative humidity.