Based on the results of long-term measurements of water temperature in three dimictic lakes, we obtained estimates of the average effective coefficients of vertical turbulent exchange for the period of formation of stable thermal stratification (July-August). Deep lakes were chosen as objects of this study: Onega (Russia), Kallavesi (Finland), Inari (Finland). For seasonal water temperature trends averaged over a multi-annual period the effects of horizontal heat transfer can be neglected, so the estimates were based on the analysis of the one-dimensional version of the heat transfer differential equation. The effective thermal diffusivity was calculated in three ways: based on changes in the heat content of the water column, local changes in water temperature, and also through the analysis of the amplitude and phase response of the deep layers temperature on quasi-periodic changes in the average temperature of the epilimnion. These estimates are two to four orders of magnitude greater than the value of the coefficient of molecular thermal diffusivity in water.