The degree of freedom of the Psychrometric chart is three (i.e., barometric pressure, DBT and WBT). To define the position of any point in the Psychrometric chart is three because the Psychrometric chart is drawn to represent the thermodynamic properties of moist air. Moist air contains water vapour and dry air. According to Gibbs phase rule
P+F=C+2
For Moist Air
Number of phases P=1
Number of components in moist air C=2
P+F=C+2
1+F=2+2
F=3