The rise of an isolated dry thermal bubble in a quiescent unstratified environment is a prototypical natural convective flow. This study considers the rise of an isolated dry thermal bubble of ellipsoidal shape (elliptical in both horizontal and vertical cross sections). The azimuthal asymmetry of the bubble allows the vorticity tilting mechanism to operate without an environmental wind. The dry Boussinesq equations of motion are solved analytically as a Taylor series in time for the early time behavior of the bubble (involving derivatives of up to the third order in time). The analytic results are supplemented with numerical simulations to examine the longertime behavior. The first nonzero term in the Taylor expansion for the vertical vorticity is a third-order term, and appears as a four-leaf clover pattern with lobes of alternating sign. The horizontal flow associated with this vorticity pattern first appears as a sheared stagnation point-type flow, but eventually organizes into vertical vortices that fill the bubble. The vortices induce large structural changes to the bubble and eventually reverse the sense of the azimuthal asymmetry. 1. Introduction and background Natural (free) convective flows abound in nature and technology. Examples include flows induced by lit cigarettes, computer CPUs, electronic circuitry, radiator fins, charcoal grills, and the heated ground. The structure of the convective atmospheric boundary layer is dependent, in part, on the statistical structure of individual and mutually interacting convective elements, and on the interactions between convective elements and the environmental wind. Many of these and other natural convective flows are azimuthally asymmetric. However, because even the most idealized natural convective entities are intrinsically nonlinear (fluid inertia and the coupling between dynamical and thermodynamical variables are essential aspects of the problem), they have been difficult to analyze theoretically without considering their ensemble characteristics to be axisymmetric or slab-symmetric. The present study is one of the first to focus on the impact of asymmetrical geometry (ellipticity in both horizontal and vertical cross sections) on the behavior of an isolated thermal bubble. The azimuthal ellipticity allows the vorticity tilting mechanism to operate even in the absence of wind shear. The subsequent development of vertical vortices and associated