Why Hot Water Freezes Faster Than Cold Water

The main factors appear to be:

• Evaporation - More hot water will evaporate than cold water, thus reducing the amount of water remaining to be frozen. Mass measurements lead us to believe this is an important factor when chilling water in open containers, though it isn't the mechanism that explains how the Mpemba Effect occurs in closed containers.
• Supercooling - Hot water tends to experience less of a supercooling effect than cold water. This makes it more likely to become solid when it reaches the freezing point of water.
• Convection - Water develops convection currents as it cools. Water density usually decreases as temperature increases, so a container of cooling water typically is warmer on top than on the bottom. If we assume water loses most of its heat across its surface (which may or may not be true, depending on the conditions), then water with a hotter top would lose its heat and freeze faster than water with a cooler top.
• Dissolved Gases - Hot water has less capacity to hold dissolved gases than cold water, which may affect its rate of freezing.
• Effect of the Surroundings - The difference between the initial temperatures of two containers of water may have an effect on the surroundings that could influence the rate of cooling. One example would be warm water melting a pre-existing layer of frost, permitting a better cooling rate.