The dynamic effects of a cumulus ensemble on a quasi-geostrophic weather system in the midlatitude region are examined, in addition to the heating due to release of latent heat. It has been found that the effects of the eddy vertical advection of momentum, and the eddy dynamic pressure due to the eddy kinetic energy associated with cumulus clouds are rather small. The only important dynamic effect comes from the ability of clouds to produce a horizontal eddy flux of vertical vorticity. In the momentum equation, this effect appears in the form of an eddy Coriolis force, which may be obtained from the horizontal eddy vorticity flux by rotating it by 90" clockwise. In the vorticity equation, it appears in the form of generation of positive vorticity by convective elements in the lower levels, and generation of negative vorticity in the upper levels.Using Ekman-layer pumping theory as the closure condition, the effects of cumulus convection on baroclinic instability are investigated. It has been found that the growth rate of a baroclinically unstable wave is increased appreciably by the cloud scale processes.Tellus 34 (1982), 1 0040-2826/82/010063-11%02.50/0 0 1982 Munksgaard, Copenhagen