We study two processes which may govern interhemispheric differences in the temperature of the summertime middle atmosphere. The first is the direct radiative effect arising from the eccentricity of Earth's orbit. The second factor is the difference in gravity-wave filtering due to the hemispheric asymmetries in the summertime mean winds of the troposphere and lower stratosphere. Using two different gravity-wave drag parameterizations and a zonal wind climatology, we find greater gravity wave induced acceleration of the zonal flow in the southern summer lower stratosphere, which leads to weaker gravity wave drag in the southern upper mesosphere. Using a two-dimensional chemical-dynamical model, we evaluate the temperature changes caused by these drag differences and compare them with those caused by direct radiative asymmetry. The radiative asymmetry peaks in the upper stratosphere but is nonnegligible between 20 and 80 km. The dynamical asymmetry has a primary contribution in the upper mesosphere and a secondary contribution in the lower stratosphere. Overall, our results support the idea that poleward of 30¿ the southern middle atmosphere is warmer than the north by 3--8K between 20 and 85 km. As a result, our model suggests that the relative humidity of the northern summer mesopause region is greater than in the south. This implies that mesospheric clouds (PMCs and NLCs) should be more frequent and more extensive in the north than in the south.