The MINOS experiment has observed a rise in the underground muon charge ratio r μ = μ + / μ - . This ratio can be related to the atmospheric production ratios of π + / π - and K + / K - . Our analysis indicates that the relevant variable for studying the charge ratio is E μ surface cos θ , rather than E μ surface . We compare a simple energy dependent parameterization of the rise in the charge ratio with more detailed previously published Monte Carlo simulations and an analytical calculation. We also estimate the size of two previously neglected effects in this context: the charge sign dependency of the d E/d x in rock, and the energy dependence of heavy primaries on the derived K + / K - ratio.