This is the ratio of the acceleration due to gravity and the specific heat of the air, according to (3.53) of Reference 2.
References
Wallace, John M., and Peter Victor Hobbs. Atmospheric Science: An Introductory Survey. 2nd ed. International Geophysics Series,
https://pds-atmospheres.nmsu.edu/education_and_outreach/encyclopedia/adiabatic_lapse_rate.htm