Aurora is a high-power KrF laser system now being constructed for inertial confinement fusion (ICF) studies at the Los Alamos National Laboratory. It will use optical angular multiplexing and serial amplification by electron-beam-driven KrF amplifiers to deliver a stacked, multikilojoule 5-ns-duration laser pulse to ICF targets. The requirements of angular multiplexing KrF lasers at the multikilojoule level dictate path lengths on the order of 1 km. The inherent complicated path crossings produced by angular multiplexing and pulse stacking do not allow isolation of individual beam lines either for evacuation or the control of air motion, so the optical quality of the long beam paths must be controlled. Propagation of the 248-nm light beams over long paths in air is affected by scattering (Rayleigh, aerosol, Raman), absorption by atmospheric gases, thermal gradients and turbulence, beam alignment, and control and optical component figure errors. Practical experience indicates that good beam quality cannot be obtained for modest path lengths (several tens of metres) unless an environmentally isolated beam tube is employed. We examine how these mechanisms affect beam propagation in the Aurora system and report on experiments performed to characterize air as a practical propagation medium.