Relativistic Runaway Electrons Avalanches (RREAs) development depends on the applied electric field and the environment air density. This dependency controls the RREA exponential growth length scale. The RREA development affects the bremsstrahlung excess occurring due to the passage of charged particles through the thundercloud’s electric fields, the gamma-ray glow. We used Monte Carlo simulations to develop an empirical model showing the RREA behavior in a realistic atmospheric density profile. The new formulation shows how the density variation modulates the electron population under electric field strengths near the RREA electric field threshold. The model limits the initial RREA altitude range as a function of the electric field strength. The new model is valid between $\sim$ 0.6 and $\sim$ 18 km, covering the relevant heights to investigate the generation of ground-detected gamma-ray glows.