Relative humidity is defined as the ratio of the amount of water vapor in the air to the maximum amount of water vapor that could be present if the water vapor were at saturation conditions, and it changes with temperature.

• A. The ratio of the amount of water vapor in the air to the maximum amount of water vapor that could be present if the water vapor were at saturation conditions; it changes with temperature.
• B. The total atmospheric pressure from all gases.
• C. The mass of water vapor per cubic meter of air.
• D. The percentage of air that is water vapor by volume.

Answer: (A)

Relative humidity is the ratio of the actual amount of water vapor in the air to the maximum amount the air could hold at the current temperature (its saturation point). This definition captures why RH changes with temperature: warmer air can hold more water vapor, so the same amount of vapor can be farther from or closer to saturation as temperature rises or falls. In practice, RH is often expressed as a percentage of that saturation value.

This is the best description because it ties together both the actual vapor present and the air’s capacity to hold vapor at a given temperature. The other ideas describe different moisture concepts: total atmospheric pressure, the mass of water vapor per unit volume (absolute humidity), or simply the fraction by volume of air that is water vapor, none of which match the way relative humidity is defined.