Psychrometric definition — choose the correct term. The following ratio is considered at the same temperature and pressure: actual mass of water vapour present in 1 kg of dry air divided by the mass of water vapour that 1 kg of dry air would hold at saturation. What is this quantity called?

Introduction / Context:
In heating, ventilation, and air-conditioning (HVAC) and in drying operations, engineers routinely compare how much moisture air actually contains with how much it could contain at saturation under the same temperature and pressure. Multiple names circulate in practice, so selecting the correct one is essential for correct psychrometric calculations and chart work.

Given Data / Assumptions:

• Temperature and total pressure are fixed for the comparison.
• Air is treated as a mixture of dry air and water vapour.
• The definition compares an actual moisture content to the saturated moisture content for the same state.

Concept / Approach:
Relative humidity (RH) is defined as the ratio of the actual amount of water vapour present to the maximum amount possible at the same temperature and pressure. While RH is often introduced via partial pressures (p_v / p_vs), it can equivalently be expressed using masses at the same state, provided the comparison is made on a consistent basis (e.g., per kilogram of dry air). This directly matches the statement in the stem.

Step-by-Step Solution:
Identify the ratio in the stem: (actual mass of vapour per kg dry air) / (saturated mass of vapour per kg dry air at same T, p).Recognize this as the saturation fraction, i.e., relative humidity.Alternative expression often used: RH = p_v / p_vs at the same temperature (for ideal-gas behaviour).Therefore, the correct term is relative humidity.

Verification / Alternative check:
On a psychrometric chart at fixed temperature, moving from an unsaturated state to the saturation curve increases the moisture content until the ratio equals 1.0 (or 100%). That is exactly the definition of relative humidity.

Why Other Options Are Wrong:
Humidity ratio is the absolute mass of vapour per mass of dry air (not a ratio-to-saturation). Absolute humidity is mass of vapour per unit volume of moist air. Degree of saturation is a different ratio comparing actual humidity ratio to the humidity ratio of saturated air at the same temperature but adjusted for total pressure effects; it is not identical to RH.

Common Pitfalls:
Confusing relative humidity (a fraction or percentage) with humidity ratio (a mass ratio independent of saturation). Another pitfall is mixing partial-pressure and mass-based definitions without keeping temperature and pressure constant.

Final Answer:
relative humidity