There are more dust devils (dd; inferred from dust devil tracks, or ddt) in the southern hemisphere than in the north. Ddt suggest that the dd season starts in late spring (Ls = 60 degrees and 240 degrees, northern and southern hemispheres, respectively) and continues through the summer into mid fall (Ls = 210 degrees and 30 degrees, northern and southern hemispheres, respectively). However, the ddt density in the southern hemisphere averages similar to 0.6 ddt/km(2), while the northern hemisphere averages similar to 0.06 ddt/km(2), or an order of magnitude less. This is attributed to the observation that in southern summer the surface receives 40% more solar energy for atmospheric motion and dd formation than the surface in the northern summer, due to the eccentricity of Mars' orbit.