The onset of convection, in a water layer continuously formed by melting ice from above, has been obtained by the experimental determination of the water layer depthdat which the mode of heat transfer changes from conduction to convection. This was accomplished both by determining the time at which the temperature profile of the water layer started to deviate from linear distribution and by locating the inflection point in the curve relating water‐ice interface position and time. Experimental values ofd(either for melting from above or below) were found with a mean deviation of 14.1% from those by theoretical analysis of a similar problem. From the present and previous investigations [Phys. Fluids 11, 1263 (1968)], it can be concluded that the critical Rayleigh number for a horizontal layer of fluid with a density inversion is not a single value as for a normal fluid with a monotonic density‐temperature relationship, but varies with boundary temperatures.