We examine the gamma–gamma and lognormal distributions as they apply to the irradiance at a point detector produced by partially coherent beams propagating horizontally through atmospheric turbulence. Our investigation compares the probability density functions and probability of fade predicted by the distributions with results from a wave-optics simulation developed for partially coherent beam propagation. For a partially coherent beam that is not too far removed from a coherent beam, we find the wave-optics results tend to the gamma–gamma model for the weak fluctuation regime and the results are closer to the lognormal model for the strong fluctuation regime. We observe that increasing the initial beam size/Fried parameter ratio ( $w_0/r_0$ ) or shortening the coherence length ( $l_c$ ) tends to narrow the probability density profile produced by the simulation.