Psychrometrics — ratio based on the same volume at the same state What do we call the ratio of the actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume when saturated at the same temperature and pressure?

Introduction / Context:
Several moisture measures exist for moist air, including humidity ratio, relative humidity, absolute humidity, and degree of saturation. Selecting the correct one requires careful attention to whether the definition uses mass-per-mass, mass-per-volume, or a ratio to saturation at the same state.

Given Data / Assumptions:

• Comparison is for the same volume of moist air.
• Reference state is saturation at the same temperature and pressure.
• Ideal gas behavior is assumed for the common definitions.

Concept / Approach:
Relative humidity is the ratio of actual water vapour amount to the maximum possible at the same temperature and pressure. It can be expressed via partial pressures (p_v/p_vs) or via mass in the same volume at the same state, which is equivalent since mass is proportional to partial pressure for ideal gases.

Step-by-Step Solution:
Identify definition: ratio to saturation for the same T and p.Map to standard terms: that is relative humidity.Confirm against alternatives: humidity ratio is per kg of dry air; absolute humidity is mass per unit volume; degree of saturation differs from RH.

Verification / Alternative check:
Psychrometric charts show RH lines as fractions/percentages of saturation at the same T and p.

Why Other Options Are Wrong:
(a) humidity ratio is m_vapour per kg dry air. (c) absolute humidity is m_vapour per m^3 of moist air. (d) degree of saturation is related but not identical to RH.

Common Pitfalls:
Confusing relative humidity with humidity ratio because both use “humidity” in the name.

Final Answer:
relative humidity