abstract: We consider the three dimensional Navier-Stokes equations and we prove that for all Leray-Hopf weak solutions it is possible to characterize (up to sub-sequences) the long time average, which satisfies the Reynolds' averaged equations. Moreover, we show the validity of the Boussinesq hypothesis, without any further assumption. We also consider ensemble averages of solutions and we prove that the fluctuations continue to have a dissipative effect on the mean flow.