What is the mass of water vapour present per unit mass of air, a quantity that remains unchanged when temperature changes without adding or removing moisture, called?

Correct answer: Mixing ratio or specific humidity

Correct answer: B. Specific humidity is the mass of water vapour per unit mass of moist air, while mixing ratio is commonly defined as the mass of water vapour per unit mass of dry air. In elementary discussions these related measures are often grouped together, and both describe the actual amount of water vapour rather than the air’s capacity to hold more moisture. If an air parcel rises, expands and cools without gaining or losing water vapour, its actual vapour content can remain essentially constant even though relative humidity increases. Relative humidity is a percentage comparing present vapour to the saturation amount at that temperature, so it can change when temperature changes. C is a fixed molecular property, not atmospheric moisture content. D is not the requested measure; dew point is a temperature, not usually a mass ratio. Strictly, specific humidity and mixing ratio have slightly different denominators, so the wording should not treat them as mathematically identical.