[en] It is shown that the flow generated by a discrete thermal with line symmetry will depend on the shape of the initial temperature perturbations. For the same original heat content a temperature excess with a large aspect ratio forms a thermal which rises faster than that developing from a long shallow perturbation. For initial conditions with the same aspect ratio, the rate of rise varies inversely with the size of the (initial) zero-temperature gradient near the axis. In all cases the thermals that form the smaller angle with the vertical axis rise faster. This observation is similar to that observed in laboratory models