Adiabatic

Adiabatic processes are processes which occur without gain or loss of heat in the working fluid. The term also describes things which are impermeable to heat transfer; for example, an adiabatic boundary is a boundary that is impermeable to heat transfer. An insulated wall approximates an adiabatic boundary condition. Another example is the adiabatic flame temperature, which is the temperature that would be achieved by a flame in the absence of heat loss to the surroundings. A reversible isentropic process is also an adiabatic process.

Adiabatic heating and cooling are processes that commonly occur due to a change in the pressure of a gas. This can be quantified using the ideal gas law.

There are three different rates of adiabatic cooling for air.

1. The ambient atmosphere lapse rate, which is the rate that air cools as one goes up in altitude.
2. The dry adiabatic lapse rate, -10°C per 1000m rise.
3. The wet adiabatic lapse rate, about -6° per 1000m rise.

The first rate is used to describe the temperature of the surrounding air that the rising air is passing through, and the second and third rates are in reference to a parcel of air that is rising through the atmosphere. The dry adabatic lapse rate applies to air which is below its dew point, ie which is not saturated by water vapor, whereas the wet adabatic lapse rate applies to air which has reached its dew point. Adabatic cooling is a common cause of cloud formation.

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 (perturbation theory)

In perturbation theory, a sufficiently slow change in the Hamiltonian, called an adiabatic change, would result only in a change of eigenvalues, not eigenstates.