  Polyakov loops $L_a(T), a=3,8,...$ are shown to give the most important
nonperturbative contribution to the thermodynamic potentials. Derived from the
gluonic field correlators they enter as factors into free energy. It is shown
in the $SU(3)$ case that $L_a (T)$ define to a large extent the behavior of the
free energy and the trace anomaly $I(T)$, most sensitive to nonperturbative
effects.
