The natural circulation of near-4°C water in a vertical cavity heated from the side was studied experimentally in a 0.74-m-tall enclosure. The results of a scale analysis were used in order to correlate the overall wall-to-wall heat transfer rate measured experimentally in the Rayleigh number range 108–1011. An analytical similarity solution was obtained for the slender enclosure limit. In that limit the flow pattern is one of incomplete vertical penetration, with stagnant maximum-density water filling the lower end of the vertical enclosure.