The differences in the ionization rates for RF induced atmospheric breakdown between the analytic formula given by Gurevich et al. (1978) and recent numerical simulations Short et al., 1991; Tsang et al., (1991) were examined and clarified. An updated analytic formula for the ionization rate of air, which includes corrections due to kinetic effects of the electron distribution function, was derived. The formula is valid for arbitrary ratios of RF to electron-neutral collision frequency. ¿ American Geophysical Union 1993