The weight of water vapor per unit weight of air is called which term?

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

Answer: (D)

The idea here is moisture content expressed as a mass ratio. Specific humidity is the mass of water vapor in a given mass of moist air, usually expressed as kilograms of water vapor per kilogram of moist air. It is a weight-based measure that compares how much water vapor is present to the total air mass, so it remains a consistent way to describe moisture content even as the air’s volume or conditions change.

This differs from absolute humidity, which is the mass of water vapor per unit volume of air (grams per cubic meter) and varies with temperature and pressure. Relative humidity is a percentage expressing how close the air is to saturated at the current temperature, not a mass ratio. Vapor pressure is the pressure contributed by the water vapor itself, not a ratio to the air’s mass.

So the term that matches “weight of water vapor per unit weight of air” is specific humidity.