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:
BTU/lb of dry air.
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.
One way to measure the effect of moisture present in an air-vapor mixture is via wet-bulb temperature. It is measured using a thermometer with a wetted bulb moving rapidly through the air in order to facilitate evaporation. If the air is relatively dry, then evaporation will occur rapidly, resulting in a cooling of the thermometer bulb. As a result of the evaporative cooling, the measured temperature will be substantially lower than the dry-bulb temperature.
The difference between the corresponding dry-bulb and wet-bulb temperatures is termed the wet-bulb depression. If the air is heavily moisture-laden, evaporation will be minimal and the wet-bulb depression will be small. In completely saturated air, the wet-bulb temperature will equal the dry-bulb temperature.
Visualizing Sensible and Latent Heat
In order to visualize sensible and latent heat, let's consider the example of one pound of water (see figure below). First, let's cool it down to a 0 degrees Fahrenheit block of ice. The thermal energy required to heat the 0F block of ice up to 32F is 16 British Thermal Units (BTU; basically, the equivalent of one kitchen match of heat). This is sensible heat — the temperature of the ice was increased.
What happens to water at 32F? It achieves its melting point and will undergo a phase change from solid to liquid as additional thermal energy is provided. In order to fully change 32F ice to 32F water is 144 BTUs. The ice would take on all of that thermal energy and yet it would not increase in temperature one bit — that's latent heat.
In order to heat the pound of 32F water up to 212F, 180 BTUs will be required. But at 212F, we reach the boiling point of water and once again phase change will occur as additional thermal energy is taken on. However, converting water to vapor requires a tremendous amount of energy: 970 BTUs.
Once the water is completely evaporated, the vapor can take on additional energy inputs. Its sensible heat will continue to increase. The pressure of the water vapor will also increase as the additional thermal energy increases molecular activity.
Sensible and latent heat graphically depicted for a pound of water.
Illustration courtesy of Daniel Overbey.