(also adiabat), a line depicting a stable adiabatic process (that is, a process in which no heat is exchanged with the surroundings) on a diagram of state. The simplest adiabatic curve is that for an ideal gas. For this case, the equation of the adiabatic curve is pvy - const, where ρ is the pressure exerted by the gas, ν is the specific volume of the gas, and y is the adiabatic index, which is constant for a given gas and is equal to the ratio of the specific heats of the gas at constant pressure cp and at constant volume cv : γ = cp /cv. For monatomic gases (argon, neon, etc.), γ = 1.67 at ordinary temperatures; for diatomic gases (hydrogen, nitrogen, oxygen, etc.), γ = 1.4. At very low temperatures (near absolute zero) and at high temperatures (above 1000°C), the slope of the curve is somewhat different, since y depends on both temperature and pressure.
For equilibrium (reversible) adiabatic processes the entropy is characteristically constant. For this reason the adiabatic curve can also be called an isoentropic curve.