Absolute humidity measures what?

• A. The mass of water vapor per unit weight of moist air
• B. The mass of water vapor per unit volume of moist air
• C. The relative amount of water vapor relative to saturation
• D. The total mass of water vapor in the air

Answer: (B)

Absolute humidity is the amount of water vapor in a given volume of moist air. It is the mass of water vapor per unit volume, usually expressed in grams per cubic meter. This directly tells you how much water vapor is present in a specific portion of air, independent of weather or sensation. It differs from specific humidity, which uses mass of vapor per unit mass of air, and from relative humidity, which compares the current water vapor amount to what the air can hold at that temperature (saturation). So describing it as mass per unit volume matches the concept, while the other phrases refer to the other humidity measures or to total mass without considering volume.