It is shown that the ionization mechanism due to the direct collision of molecules replaces the mechanism of associative ionization in the hypersonic flow of air around a blunt body as the density diminishes. The value of the Reynolds number for which this transition occurs is found. The results of computations of a viscous nonequilibrium shock layer are compared with experimental results of American researchers. Good agreement is obtained between results on the distribution of the electron concentrations in the shock layer. It is shown that the barodiffusion process exerts a great influence on the distribution of the electron concentration in the shock layer at low Reynolds numbers.