How Can Ice Emerge at 0 °C?

The classical nucleation theory shows that bulk water freezing does not occur at temperatures above approximate to -30 degrees C, and that at higher temperatures ice nucleation requires the presence of some ice-binding surfaces. The temperature and rate of ice nucleation depend on the size and level of complementarity between the atomic structure of these surfaces and various H-bond-rich/depleted crystal planes. In our experiments, the ice nucleation temperature was within a range from -8 degrees C to -15 degrees C for buffer and water in plastic test tubes. Upon the addition of ice-initiating substances (i.e., conventional AgI or CuO investigated here), ice appeared in a range from -3 degrees C to -7 degrees C, and in the presence of the ice-nucleating bacterium Pseudomonas syringae from -1 degrees C to -2 degrees C. The addition of an antifreeze protein inhibited the action of the tested ice-initiating agents.