Understanding the lines plotted on a psychrometric chart is crucial:

The Psychrometric Chart: An Essential Guide to Understanding Air Properties and HVAC Design

Psychrometric charts are a powerful tool for analyzing and understanding the complex relationships between air properties. By mastering the use of psychrometric charts, engineers, scientists, and technicians can design and optimize systems, analyze indoor air quality, and predict weather patterns. With practice and experience, users can unlock the full potential of psychrometric charts and make informed decisions in various fields.

But the chart told her more. The enthalpy lines—running diagonally from upper left to lower right, marked in BTUs per pound—gave her the total heat hiding in the air, sensible and latent together. She traced along the constant enthalpy line to the saturation curve (the leftmost boundary, where relative humidity hits 100%, the edge of fog and rain). The number there told her how much energy she’d need to wring the water out.