An air parcel is a theoretical concept representing an imaginary volume of air. The air parcel is assumed to move without mixing immediately with the surrounding environment, allowing scientists to model how atmospheric processes affect temperature and moisture. The cooling of an air parcel is a fundamental process in weather and climate, often directly linked to its vertical movement as it rises or sinks. Understanding how an air parcel loses heat is necessary for predicting cloud formation, precipitation, and atmospheric stability.
Cooling Through Expansion and Lifting
The most significant cause of cooling for an air parcel is its upward movement, a process known as adiabatic cooling. As an air parcel ascends in the atmosphere, the surrounding atmospheric pressure steadily decreases. Since the parcel’s internal pressure must match the external pressure, the parcel expands dramatically as it rises.
This expansion requires the air parcel to perform mechanical work against the lower pressure of the surrounding air. The energy needed for this work is drawn directly from the internal kinetic energy of the air molecules within the parcel itself. This expenditure of internal energy results in a measurable decrease in the air parcel’s temperature, even though no heat energy has been exchanged with the environment.
This process is termed “adiabatic” because it occurs without the transfer of heat across the parcel’s boundary. For an unsaturated air parcel, this cooling occurs at a predictable and constant rate, known as the Dry Adiabatic Lapse Rate (DALR). This rate is approximately 9.8 degrees Celsius for every 1,000 meters the parcel is lifted.
Cooling Through External Heat Transfer
Cooling can also occur through diabatic processes, which involve the physical transfer of heat energy between the air parcel and its surroundings. One such process is radiational cooling, where the air parcel emits longwave radiation into space. Atmospheric gases, particularly water vapor and carbon dioxide, constantly emit infrared energy, leading to a cooling effect that is most pronounced at night.
Radiational cooling is especially effective when the skies are clear and the air is relatively dry, allowing longwave radiation to escape directly through the atmosphere’s “window” into outer space. This constant emission of energy acts to lower the parcel’s temperature over time without requiring any vertical movement.
The air parcel can also lose heat through conduction and convection. Conduction occurs when a warmer air parcel rests directly against a colder surface, transferring its heat to the surface. Since air is a poor conductor, this process is generally limited to the air layer immediately adjacent to the cold surface. Convection and turbulent mixing further contribute to cooling when a warm air parcel mixes with the surrounding, cooler air, distributing its thermal energy across a larger volume.
Cooling Through Phase Changes
A third mechanism for cooling an air parcel involves the energy absorbed during a change in the state of water, a process tied to latent heat. Latent heat is the energy absorbed or released when a substance changes phase, and its absorption causes a significant cooling effect on the air. The primary way this occurs is through evaporation, where liquid water converts into water vapor.
When liquid water is present, the transition to a gaseous state requires a substantial intake of energy. This necessary energy, the latent heat of vaporization, is absorbed from the air molecules within the parcel. This converts the air’s measurable sensible heat into latent heat stored in the vapor, causing the temperature of the air parcel to drop noticeably.
Evaporative cooling is highly effective. A similar cooling effect can happen through sublimation, where ice or snow converts directly into water vapor without first melting into liquid water. Both evaporation and sublimation absorb thermal energy from the air, causing the air parcel to cool down.