Experimental data on free convection of water in the region of maximum density are interpreted by a theoretical model. The agreement between our theory and experimental data is excellent.
Typical arrests in the temperature versus time curves and loops in warming-cooling cycles indicate large effects on convection generated by small changes in the space-time density profiles. These observations are fully explained by an extended theoretical approach which takes into account complex phenomena occurring in the boundary layer region. The comparison between theory and experiments indicates how the convective motion propagates from the boundary layer to the internal region of the fluid (central nucleus). Thanks to the extreme sensitivity of the temperature behaviour to relatively small density variations, a complete information about density profile around 4O C can be obtained by a quite simple experimental apparatus. Experimental data at temperature ranges between 0" C and 8O C can "feel" the asymmetry in the density curve around 4" C which is of about 8 parts per million. Our results can bring some light on the experiments performed by Azouni about the water hysteresis loop around 4' C.