There is energy in the air all around us. Some of it exists in the form of thermal energy that one can sense – heat in air that impacts one’s perception of thermal comfort. But there is also an energy embodied in the vapor present in the air. While this “hidden” heat is not experienced as warmth, it certainly has a bearing on thermal comfort as well.
In order to distinguish between these two types of heat in the air, one must determine the air’s enthalpy.
Enthalpy is a term used to describe the total amount of heat present in an air-vapor mix. It is the sum of both the sensible and latent heat present in the air. Recall that sensible heat is the “dry” heat in the air related to dry-bulb temperature (think glowing coils on an electric cooking range); while latent heat is the “wet” heat release into the air as water phase-changes to vapor (think vapor released to the air as a tea kettle’s contents boil atop said cooking range). (See Thermal Dynamics: Visualizing Sensible Versus Latent Heat for more information on sensible vs latent heat.)
The units for sensible heat, latent heat, and enthalpy are all the same: BTUs per pound of dry air (BTU/lb).