From 2010.igem.org

The nucleation is the formation of the new centers from which spontaneous growth can occur

Various processes are involved in the formation of solid phase of the water, among others, the interaction between ions or molecules leads to the formation of nucleus. The nucleation is the formation of the new centers from which spontaneous growth can occur; this processes determines the size and distribution of produced crystals. Large crystals may eventually be formed from fine crystals by a process called ripening. there are a lot of ions and other particles Inside the cells. So when the temperature decreases the nucleation process arise in the cytoplasm.