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Specific heat ratio of various gases
Gas	°C	k		Gas	°C	k
H2	−181	1.597		Dry Air	20	1.40
	−76	1.453			100	1.401
	20	1.41			200	1.398
	100	1.404			400	1.393
	400	1.387		CO2	0	1.310
	1000	1.358			20	1.30
	2000	1.318			100	1.281
He	20	1.66			400	1.235
N2	−181	1.47		NH3	15	1.310
	15	1.404		CO	20	1.40
Cl2	20	1.34		O2	−181	1.45
Ar	−180	1.76			−76	1.415
	20	1.67			20	1.40
CH4	−115	1.41			100	1.399
	−74	1.35			200	1.397
	20	1.32			400	1.394

The specific heat ratio of a gas is the ratio of the specific heat at constant pressure, $C_{p}$, to the specific heat at constant volume, $C_{v}$. It is sometimes referred to as the adiabatic index or the heat capacity ratio or the isentropic expansion factor or the adiabatic exponent or the isentropic exponent.

Either $\kappa$ (kappa), $k$ (Roman letter k) or $\gamma$ (gamma) may be used to denote the specific heat ratio:

$\kappa =k=\gamma ={\frac {C_{p}}{C_{v}}}$

where:

$C$ = the specific heat of a gas

$p$ = refers to constant pressure conditions

$v$ = refers to constant volume conditions