Ratio of specific heats
The ratio of specific heats (also known as adiabatic index), usually denoted by , is the ratio of specific heat at constant pressure to the specific heat at constant volume.
The adiabatic index always exceeds unity; for a polytropic gas it is constant. For monatomic gas , and for diatomic gases , at ordinary temperatures. For air its value is close to that of a diatomic gas, 7/5 = 1.4.
Sometimes is used instead of to denote the specific heat ratio.