Three types of heat budgets were covered in this chapter. All depend on the flow rate of energy per unit area (J m–2 s–1) into or out of a region. This energy flow is called a flux, where the units above are usually rewritten in their equivalent form (W m–2).
1) One type is a heat balance at the surface of the Earth. The surface has zero thickness — hence no air volume and no mass that can store or release heat. Thus, the input fluxes must exactly balance the output fluxes. Sunlight and IR radiation (see the Radiation chapter) must be balanced by the sum of conduction to/from the ground and effective turbulent fluxes of sensible and latent heat between the surface and the air.
2) Another type is an Eulerian budget for a fixed volume of air. If more heat enters than leaves, then the air temperature must increase (i.e., heat is stored in the volume). Processes that can move heat are advection, turbulence, and radiation. At the Earth’s surface, an effective turbulent flux is defined that includes both the turbulent and conductive contributions. Also, heat can be released within the volume if water vapor condenses or radionuclides decay.
3) The third type is a Lagrangian budget that follows a mass of air (called an air parcel) as it rises or sinks through the surrounding environment. This is trickier because the parcel temperature can change even without moving heat into it via fluxes, and even without having water vapor evaporate or condense within it. This adiabatic temperature change is caused by work done on or by the parcel as it responds to the changing pressure as it moves vertically in the atmosphere. For unsaturated (non-cloudy) air, temperature of a rising air parcel decreases at the adiabatic lapse rate of 9.8°C km–1. This process is critical for understanding turbulence, clouds, and storms, as we will cover in later chapters.
The actual air temperature can be measured by various thermometers. Humans feel the combined effects of actual air temperature, wind, and humidity as an apparent air temperature.