What is an Adiabatic Process?

In physics, an adiabatic process is a system that does not exchange heat with its environment. This means when the system performs work — whether movement or mechanical work — it ideally does not make its surroundings warmer or cooler. For systems involving gases, an adiabatic process usually requires changes in pressure to shift temperature without affecting the surrounding environment. In the Earth's atmosphere, air masses will undergo adiabatic expansion and cool down, or they will experience adiabatic compression, and heat up. Engineers have designed various engines with processes that are at least partially adiabatic.

An adiabatic process is a thermodynamic process in which a system does not gain or lose heat to its surrounding environment. A thermodynamic process can be understood as a measurement of energy changes within a system, taken from a beginning state to an ending state. In applications of thermodynamics, a system may be any clearly defined space with a uniform set of properties, whether a planet, an air mass, a diesel engine, or the universe. While systems have many thermodynamic properties, the important one here is temperature change, measured by heat gain or heat loss.

A change in the internal energy of a system will occur whenever that system performs work, as when a machine powered by internal combustion moves its parts. In adiabatic processes involving most atmospheric gases, such as air, compression of the gas within the system causes the gas to warm, while expansion cools it. Some steam engines have taken advantage of this process to increase pressure and thus temperature, and are considered adiabatic engines. Scientists classify adiabatic processes — from machines to weather systems — according to whether or not they are reversible to their original temperature.

Within an adiabatic process, a temperature change will occur only due to the work that it performs, but not due to heat loss to its environment. Rising air cools without losing heat to neighboring air masses. It cools because atmospheric pressure, which compresses and heats the air closer to earth's surface, decreases with altitude. As pressure on a gas is reduced, it will expand, and thermodynamic laws consider expansion to be work. When the air mass expands and performs work, it does not lose heat to other air masses that may have very different temperatures, and thus undergoes an adiabatic process.

It is nearly impossible for a perfect adiabatic system to exist, because some heat is usually lost. There are mathematical equations that scientists use to model adiabatic processes that assume a perfect system for convenience. These must be adjusted when planning actual engines or devices. The opposite of an adiabatic process is an isothermal process, where heat is transferred outside of the system to its surrounding environment. If a gas expands freely outside of a system with regulated pressure, it undergoes an isothermal process.