Two estimates of the turbulent diffusivity (i.e., the heat flux per unit gradient) in the lower stratosphere are inferred from high-resolution radar measurements and compared. First, the local heat flux (within the turbulent patches) is evaluated from the dissipation rate of turbulent kinetic energy $epsilon$k under the basic assumptions of local homogeneity and stationarity of the fluctuations. We then estimated the effective heat flux per unit gradient as the time-averaged flux for a given altitude during the measurement period (six hours), taking into account the observed turbulence intermittence. The time-averaged heat flux per unit gradient is found to be ~2 ¿ 10-2 m2s-1 typically, in good agreement with some of the weakest estimates of diffusivity already published. The observed ratio between the local and the time averaged fluxes can reach about one order of magnitude. This last result stresses the fact that turbulent diffusivity inferred from MST radars measurements cannot generally be directly interpreted as an effective diffusivity, since radar estimates, in most cases, do not take into account the turbulence intermittence.