Partial pressure of water vapor present in the air is called which term?

• A. Relative humidity
• B. Specific humidity
• C. Vapor pressure
• D. Dew point

Answer: (C)

Vapor pressure is the portion of the total air pressure that is due to water vapor. It represents the pressure water vapor would exert if it alone filled the space at the current temperature, directly indicating how much moisture is in the air. Relative humidity is a ratio comparing this actual vapor pressure to the maximum vapor pressure the air can hold at that temperature, expressed as a percentage. Specific humidity is a mass-based measure—the amount of water vapor per unit mass of moist air—so it doesn’t describe pressure. Dew point is the temperature at which air must be cooled to become saturated and begin to condense, linking moisture to temperature rather than current pressure. Therefore, the partial pressure of water vapor present in the air is vapor pressure.