The advantages of a high altitude, dry site for ground-based astronomy at infrared (IR) wavelengths are well-known: the lower temperature and pressure associated with increased altitude reduce the emissivities of both atmosphere and telescope, and a lower atmospheric absorption improves the transmission of IR radiation. The next generation of IR instruments under development (for ELTs) will open up a new discovery space, particularly in high-resolution (HR) spectroscopy, which will not have a space-based counterpart and has proven to be a powerful tool for studying all stages of stellar evolution (e.g. (e.g. Jaffle et al., 2003). I present here a summary of quantitative work into transmission-dependent aspects of HR IR spectroscopy at high and low altitudes.