The thermal and fluid kinetics of the convective layer during cooling from the free surface of a thermally stratified water by convection, latent energy transport, and radiation is studied. Laboratory experiments were performed and a Mach-Zehnder interferometer was used to measure the unsteady temperature distribution in a test cell filled with water which was previously stratified. A simple mathematical model based on a thermal energy balance is developed to predict the thickness and the mean temperature of the layer. The model predictions agreed to within 10% with the data from controlled laboratory experiments. It was found that the numerical solution to the model equations agree better with the experimental data than those based on the closed form analytical solution using the constant average surface heat flux. It was determined that the surface boundary condition and the internal physical processes of mixing and entrainment must be better understood in order to model the dynamics of the mixed layer in natural waterbodies.