Definition check: Relative humidity Relative humidity (RH) is defined as the ratio of the actual vapor pressure to the saturation vapor pressure measured under which condition?

Introduction / Context:
Relative humidity is a basic atmospheric moisture metric used in hydrology, meteorology, HVAC, and environmental engineering. It links the current moisture content to the maximum moisture air can hold at a given temperature, which controls evaporation, condensation, and human thermal comfort.

Given Data / Assumptions:

• Actual vapor pressure e reflects the partial pressure of water vapor present.
• Saturation vapor pressure es is the maximum vapor pressure air can hold at the same temperature before condensation occurs.
• RH is usually expressed as a percentage.

Concept / Approach:

The saturation capacity of air varies strongly with temperature; therefore, RH must compare e and es at the same temperature. Pressure effects are secondary for most surface applications, and “same volume” or “entire atmosphere” are not part of the definition.

Step-by-Step Solution:

Verification / Alternative check:

Psychrometric charts and Clausius–Clapeyron relations emphasize the temperature dependence of es, corroborating the definition.

Why Other Options Are Wrong:

• Same pressure only / same volume: Not the definitional condition for RH.
• Entire atmosphere / dew-point: Dew-point is where RH is 100%, not the reference condition for defining RH.

Common Pitfalls:

• Confusing absolute humidity or specific humidity with RH.
• Assuming RH alone determines comfort without considering temperature and air movement.

Final Answer:

At the same temperature.