This is the "normal" temperature measured by a standard thermometer. It runs horizontally across the bottom of the chart, usually ranging from freezing (0°C or 32°F) up to 50°C or 60°C (approx. 120°F–140°F).
The chart is unique because it allows you to identify all the properties of a specific air sample by knowing just two independent variables. How to Read a Psychrometric Chart - ProcessSensing.com psychrometric graph