Psychrometrics – Relative humidity trend How does relative humidity change as air becomes drier (i.e., moisture content decreases at roughly constant temperature)?

Introduction / Context:
Relative humidity (RH) is a key comfort and process variable defined as the ratio of the partial pressure of water vapor in air to the saturation vapor pressure at the same temperature. Understanding its qualitative behavior is crucial for HVAC control and drying operations.

Given Data / Assumptions:

• Air temperature roughly constant during the comparison.
• “Drier” means a reduction in moisture content (humidity ratio).

Concept / Approach:
At fixed temperature, saturation pressure is fixed. Reducing actual water-vapor partial pressure lowers RH because RH = p_v / p_vs. Thus, as air loses moisture while temperature remains about the same, RH declines.

Step-by-Step Solution:
Hold temperature T approximately constant ⇒ p_vs(T) is constant.Decrease moisture content ⇒ p_v drops.Compute RH = p_v / p_vs ⇒ RH decreases.

Verification / Alternative check:
On a psychrometric chart at constant DBT, moving left (lower humidity ratio) also lowers RH curves, confirming the trend.

Why Other Options Are Wrong:

• Increases/constant: contradict the definition at constant temperature.
• “First increases then decreases”: no such behavior absent temperature change.
• “Cannot be inferred”: it can be, given the stated dryness.

Common Pitfalls:
Confusing RH with absolute humidity; ignoring the effect of temperature changes—warming air without adding moisture reduces RH, while cooling raises RH even if absolute moisture is unchanged.

Final Answer:
decreases