The phase transition of a substance passing from the solid to the liquid state; melting. In meteorology, fusion is almost always understood to refer to the melting of ice, which, if the ice is pure and subjected to one standard atmosphere of pressure, takes place at the ice point of 0°C (see melting point). Additional heating at the melting point is required to fuse any substance. The specific enthalpy that must be added to fuse ice is called the latent heat of fusion and is approximately 3. 35 × 105 J kg−1.