At the moment of the explosion, the fireball is formed. The ascending, roughly spherical mass of hot, incandescent gases changes shape due to atmospheric friction and cools its surface by energy radiation, turning from a sphere to a violently rotating spheroidal vortex. A Rayleigh–Taylor instability is formed as the underneath cool air initially pushes the bottom fireball gases into an inverted cup shape. This causes turbulence and a vortex that sucks more air into its center, creating external afterwinds and cooling itself. The speed of its rotating slows down as it cools, and may stop entirely during later phases. The vaporized parts of the weapon and ionized air cool into visible gases, forming the early cloud; the white-hot vortex core becomes yellow, then dark red, then loses visible incandescence. With further cooling, the bulk of the cloud fills in as atmospheric moisture condenses. As the cloud ascends and cools, its buoyancy lessens, and its ascent slows.