Thermodynamics of an interacting trapped Bose-Einstein gas in the   classical field approximation
  We present a convenient technique describing the condensate in dynamical equilibrium with the thermal cloud, at temperatures close to the critical one. We show that the whole isolated system may be viewed as a single classical field undergoing nonlinear dynamics leading to a steady state. In our procedure it is the observation process and the finite detection time that allow for splitting the system into the condensate and the thermal cloud. 
K. Goral, M. Gajda, and K. Rzazewski