 However, it may be that indeed the homogeneous nucleation of ice at temperatures near -20 °C and above is "extremely" slow and so that whenever water freezes at temperatures of -20 °C and above this is due to heterogeneous nucleation, i.e., the ice nucleates in contact with a surface.Homogeneous nucleation is much rarer than heterogeneous nucleation.However, homogeneous nucleation is simpler and easier to understand than heterogeneous nucleation, so the easiest way to understand heterogeneous nucleation is to start with homogeneous nucleation.