### abstract

- We study experimentally temperature fluctuations in stably stratified forced turbulence in air flow. In the experiments with an imposed vertical temperature gradient, the turbulence is produced by two oscillating grids located nearby the side walls of the chamber. Particle image velocimetry is used to determine the turbulent and mean velocity fields, and a specially designed temperature probe with sensitive thermocouples is employed to measure the temperature field. We found that the ratio big [(ell _x nabla _x overline{T})^2 + (ell _y nabla _y overline{T})^2 + (ell _z nabla _z overline{T})^2big ] / < θ ^2 rangle is nearly constant, is independent of the frequency of the grid oscillations, and has the same magnitude for both, stably and unstably stratified turbulent flows, where ℓi are the integral scales of turbulence along x, y, z directions, overline{T} and θ are the mean and turbulent fluctuations components of the fluid temperature. We demonstrated that for large frequencies of the grid oscillations, the temperature field can be considered as a passive scalar, while for smaller frequencies the temperature field behaves as an active field. The theoretical predictions based on the budget equations for turbulent kinetic energy, turbulent potential energy (∝ ⟨θ2⟩), and turbulent heat flux, are in a good agreement with the experimental results. Detailed comparison with experimental results obtained previously in unstably stratified forced turbulence is performed.