It’s the first law of thermodynamics. The Law of Conservation of Energy states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another.
This is the foundation of phase change in the physical environment. Molecular activity requires energy. The activity does not just magically happen.
When a material changes its state or phase (e.g., going from a solid to a liquid or a liquid to a gas) a large amount of heat must be absorbed (or released) in order to make the transition. This is a type of heat transfer that is distinct from conduction, convection, and radiation.
The phase change of water is a common occurrence in the built environment. Therefore, it will be referenced here as an example.
If the air in a room were completely dry, devoid of any moisture whatsoever, then its thermal properties could be described using a conventional thermometer to measure the dry-bulb temperature.
However, the terrestrial environments, completely dry air conditions virtually never exist. There is always some amount of moisture present in the air in the form of water vapor. This vapor has a significant impact on thermal comfort.
Sensible Heat vs Latent Heat
Sensible heat is the “dry” heat in the air and relates directly to the dry-bulb temperature. This is the heat that one can “sense” with a conventional thermometer, as the term suggests. For example, heat added to an air mass from the glowing coil of an electric cooking range is an example of sensible heat.
Latent heat is the “wet” heat captured in the air as water undergoes phase change from liquid to vapor via evaporation or boiling. For example, water vapor added to an air mass from a tea kettle boiling on the stove is an example of latent heat. The process is also reversible as latent heat is released when moisture is condensed out of the air mass.
Enthalpy is the sum of the sensible and latent heat in a given air-vapor mix. It is sometimes referred to as the total heat of the air. The units for sensible heat, latent heat, and enthalpy are the same:
Unlike sensible heat, latent heat is not directly “sensed” by a conventional thermometer with a dry-bulb. Latent heat can be borne out by comparing the dry-bulb temperature with the wet-bulb temperature.