A model for the evolution of an inviscid buoyant thermal into a vortex ring is developed by placing Hill's spherical vortex in a gravitational field. The atmospheric pressure gradient interacts with the density gradient at the edge of the thermal to produce vorticity. This additional effect is treated as an unsteady perturbation to Hill's steady‐state solution and causes the spherical vortex to evolve into a torus. Torus formation times are obtained in terms of the thermal radiusa, the initial rise velocityV0, and the degree of buoyancyg(&Dgr;&rgr;/&rgr;) (&Dgr;&rgr;/&rgr;) arbitrary). AsV0 → 0, the results for a buoyant bubble are recovered. However, whenV02 > ag(&Dgr;&rgr;/&rgr;), the internal circulation of the thermal changes the time scale of torus formation. Denoting the thermal density as&rgr;tand that in the atmosphere as&rgr;∞, the torus formation times are shown to be[3a&rgr;t/g(&rgr;∞ − &rgr;t)]1/2in the buoyant limit andV0/g[(&rgr;∞/&rgr;t)1/2 − 1]in the inertial (largeV0) limit. The analysis is extended to a density‐stratified atmosphere, and it is shown that the expansion of the rising thermal may eliminate the buoyant force before the torus can form. The conditions under which a torus can form are obtained in terms ofa, g, V0 , &Dgr;&rgr;/&rgr;andH, the atmospheric scale height.