Example: An air conditioner in summer. The AC coil is cold (below the dew point). As air passes over it, water condenses out. The process moves diagonally down and left, following the "Apparatus Dew Point" line towards the saturation curve. Both temperature and humidity ratio decrease.
These are slightly sloped diagonal lines indicating the volume occupied by a unit mass of dry air (cubic feet per pound). This is critical for sizing fans and ducts. psychrometric graph
The psychrometric graph typically has the following axes: Example: An air conditioner in summer