The First Law of Thermodynamics supports that the energy is conserved. Thus, the thermal energy is defined as the sum of warming or internal energy (microscopic effect) and work occurring per unit mass (macroscopic effect). For its application to the Atmosphere, the thermal energy input is given from the following mathematical expression: Δq=Cp·ΔT-(ΔP/ρ), where Δq (J·kg–1) is the amount of thermal energy you add to a stationary mass m of air, Cp (J·kg–1·K–1) is the specific heat of air at constant pressure, ΔT (K) is the induced variation of temperature, so that Cp·ΔT represents the heat transferred per unit air mass, ΔP (Pa = J·m-3) is the pressure difference and ρ (kg· m-3) is the air density.
The term Cp·T is defined enthalpy h, thus, the first term on the right side of eq. of thermodynamic first low for atmospheric applications, which is the corresponding enthalpy change is: Δh=Cp·ΔT. It is a characteristic possessed by the air.
Expressing the first law of thermodynamics for atmospheric applications in conceptual form we can state that, given a quantity Δq of thermal energy added to a stationary mass m of air, a part of this energy heats the air, increasing its internal energy, but, as air heats up, its volume expands by an amount ΔV and pushes against the surrounding atmosphere, which responds with an equal and opposite pressure P that we can assume constant. Therefore, a part of the thermal energy introduced does not go to heat the air, but goes into macroscopic movement.