The numerical plasma torch model of D. A. Scott, P. Kovitya, and G. N. Haddad [J. Appl. Phys.66, 5232 (1989)], in which both the arc and plume regions are included in the computational domain, has been extended to treat the effect of the mixing of the plasma gas with the ambient gas. Both laminar diffusion and turbulent mixing are considered. The predictions of the model are compared with laser‐scattering measurements of the temperature distribution in the plume of a plasma torch, for the case in which the plasma gas is argon and the ambient gas is air at atmospheric pressure. Good agreement is found between the measurements and the predictions of the model. The rapid decay in plume temperature away from the exit of the torch nozzle is shown to be mainly due to cooling by air entrained by turbulent mixing. TheK‐&egr; turbulence model is found to adequately approximate the turbulence phenonema involved.