Ice begins to melt when its surroundings rise above its freezing point, that being 32 degrees Fahrenheit (0 degrees Celsius). Depending on the salt content and the presence of other substances, that freezing point may be an even lower temperature.
Several different factors can also cause ice to begin melting, even at very low temperatures. Adding salt or even sugar causes normally stable ice to begin melting, either until the added substance is sufficiently diluted or the surrounding temperature lowers to the freezing point of the new solution. Convection currents of either air or water can also speed the rate at which ice melts.