Once you learn to see the air through these lines, you will never look at a cloudy window or a sweat glass of iced tea the same way again. You are seeing psychrometrics in action. Standard psychrometric charts are published by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) and are available for various altitudes.
Air is invisible. Yet, for heating, ventilation, and air conditioning (HVAC) engineers, it is a complex mixture of gases, water vapor, and energy. To visualize this invisible world, engineers rely on one of the most elegant and powerful tools ever developed: The Psychrometric Chart . psychrometric graph
These are straight vertical lines (or horizontal, depending on the chart's orientation). They represent the sensible heat level of the air—the heat you can feel. Once you learn to see the air through
This is the temperature at which the air becomes saturated (100% RH) if cooled at constant pressure. To find it, move horizontally left from your point to the saturation curve. Air is invisible
These are sweeping parabolic curves. Each curve represents a percentage of saturation (0% to 100%). The 100% RH line is the Saturation Line . Air cannot hold more moisture beyond this line; it will start to condense (dew, fog, rain).