Dimensionless ratio of the gas density \rho to the air density \rho_{air}, both taken at reference conditions (normally at STP, otherwise should be specified in the context):
| (1) | \gamma_g = \frac{\rho_g}{\rho_{\rm air}} |
See also
Natural Science / Physics / Fluid density / Specific gravity
[ API gravity ][ Oil specific gravity (γo) ]