# Ratio of specific heats

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The ratio of specific heats (also known as adiabatic index), usually denoted by $\gamma$ is the ratio of specific heat at constant pressure to the specific heat at constant volume
$\gamma = \frac{C_p}{C_v}$
The adiabatic index always exceeds unity; for a polytropic gas it is constant. For monatomic gas $\gamma=5/3$, and for diatomic gases $\gamma=7/5$, at ordinary temperatures. For air its value is close to that of a diatomic gas, 7/5.