The Mpemba effect is popularly summarized by the statement that "hot water can freeze faster than cold water", and has been observed experimentally since the time of Aristotle; however, there remains no definitive explanation for the effect. Here, we consider experimentally and theoretically the freezing of water in a rectangular vessel, with a view to investigating natural convection as a possible mechanism. The experimental and theoretical results are, in general, found to agree well; however, in combination, the results suggest that, whereas natural convection gives the correct general timescale for freezing, supercooling adjusts the actual time required. Moreover, the effect of supercooling leads to a spread in the experimental freezing times, giving results that constitute evidence of the occurrence of the Mpemba effect, even though the model results by themselves do not. (C) 2014 Elsevier Ltd. All rights reserved.