A thermodynamic process in which no heat enters or leaves a system during expansion or compression of the fluid composing the system. An example of an adiabatic process is the vertical flow of air in the atmosphere; air expands and cools as it rises, and contracts and grows warmer as it descends. Another example, when an interstellar gas cloud expands or contracts.
Adiabatic changes are usually accompanied by changes in temperature. In most situations, truly adiabatic changes must take place in short time intervals so that the heat content of the system remains unchanged, or else the system must be perfectly insulated (a practical impossibility). The generation of heat when a gas is rapidly compressed, as in a piston, is approximately adiabatic.
Adiabatic changes are usually accompanied by changes in temperature. In most situations, truly adiabatic changes must take place in short time intervals so that the heat content of the system remains unchanged, or else the system must be perfectly insulated (a practical impossibility). The generation of heat when a gas is rapidly compressed, as in a piston, is approximately adiabatic.
Adiabatic heating and cooling:
Adiabatic changes in temperature occur due to changes in pressure of a gas while not adding or subtracting any heat. In contrast, free expansion is an isothermal process for an ideal gas.
Adiabatic heating occurs when the pressure of a gas is increased from work done on it by its surroundings, i.e. a piston. Diesel enginesrely on adiabatic heating during their compression stroke to elevate the temperature sufficiently to ignite the fuel.
Adiabatic heating also occurs in the Earth's atmosphere when an air mass descends, for example, in a katabatic wind or Foehn wind flowing downhill.
Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings. Adiabatic cooling does not have to involve a fluid. One technique used to reach very low temperatures (thousandths and even millionths of a degree above absolute zero) is adiabatic demagnetisation, where the change in magnetic field on a magnetic material is used to provide adiabatic cooling. Adiabatic cooling also occurs in the Earth's atmosphere with orographic lifting and lee waves, and this can form pileus or lenticular clouds if the air is cooled below the dew point.
Rising magma also undergoes adiabatic cooling before eruption.
Such temperature changes can be quantified using the ideal gas law, or the hydrostatic equation for atmospheric processes.
No process is truly adiabatic. Many processes are close to adiabatic and can be easily approximated by using an adiabatic assumption, but there is always some heat loss; as no perfect insulators exist.
