Latent heat is the heat energy, measured in calories, absorbed or released when water changes from one state to another.
There are three states of water:
1. Water vapor (gas)
2. Water liquid (cloud, mist, fog, rain, etc.)
3. Water solids (ice)
When water changes to a higher energy state, i.e., from ice to liquid to vapor, it absorbs/uses latent heat energy (from the surrounding atmosphere/properties). And when it moves to a lower energy state, i.e., from vapor to liquid to ice, it releases latent heat energy (into the surrounding atmosphere/properties). This can be important in certain forms of airframe ice formation. For example, 80 calories of latent heat is released with the change of state of 1 g of water to ice. If this water were supercooled to say —20°C, then it would already have 20 calories less latent heat energy than it ought to have at 0°C. When it then reverts to ice from water, this is made up by the latent heat being released by the change of state of the water to ice. Therefore, the 20 calories needed will change 20/80, or K, of the water to ice. The remaining % of the water runs back along the airframe, rapidly cooling and freezing as a clear sheet of ice.