Psychrometrics: choose the correct definition of relative humidity At a given temperature and pressure, select the expression that correctly defines relative humidity of moist air. Choose the correct statement.

Concept/Approach
Relative humidity φ is defined as the ratio of the actual water vapour content to the maximum possible (saturation) water vapour content at the same temperature and pressure.

Equivalent forms
φ = p_v / p_vs = ρ_v / ρ_vs at the same T and p, which corresponds to comparing equal volumes.

Why option D is correct
Option D states the ratio using equal volumes at the same T and p, matching φ = ρ_v/ρ_vs.

Why others are incorrect

• Option A defines absolute humidity (kg/m³).
• Option B defines humidity ratio/specific humidity (kg/kg dry air).
• Option C compares per unit mass of dry air; that ratio equals the ratio of humidity ratios, not the standard definition of relative humidity.

Final Answer
Ratio of actual mass of water vapour in a given volume of moist air to that in the same volume of saturated air at the same T and p