Psychrometric chart reading: On a standard psychrometric chart, the constant relative humidity lines are:

Introduction / Context:
Accurate reading of a psychrometric chart is fundamental for HVAC design, cooling tower analysis, and dehumidification calculations. Each family of lines corresponds to a different air property.

Given Data / Assumptions:

• Standard psychrometric chart at 1 atm.
• Dry-bulb temperature plotted on the abscissa; humidity ratio on the ordinate.
• Other property lines (wet-bulb, RH, enthalpy, specific volume) are overlaid.

Concept / Approach:
Constant relative humidity (RH) lines are curved, intersecting dry-bulb verticals and trending toward the saturation curve (100% RH). Their curvature reflects Clausius–Clapeyron behaviour of water vapour pressure versus temperature.

Step-by-Step Solution:

Verification / Alternative check:
Consult any standard chart; the geometry is consistent across references though exact spacing is non-uniform due to property equations.

Why Other Options Are Wrong:

• Vertical/horizontal descriptors apply to DBT and humidity ratio, not RH.
• “Horizontal non-uniform” describes enthalpy or wet-bulb trends poorly; RH is not horizontal.

Common Pitfalls:
Mistaking wet-bulb lines (near straight slanted) for RH; always locate the saturation curve first to orient the chart.

Final Answer:
Curved lines (approximately concave to the humidity ratio axis)