Upward Motion | Upward Motion No Upward Motion ’ Figure 2. Sketch of a Torus Cloud from a Device After Burst. time, during the rise of the cloud, the only external force operating against the cloud is that of atmospheric stability. This is of import- ance since energy is required to sustain the cloud rise, and this energy | must come from the thermal energy of the fireball. When the cloud reaches its greatest height, it is said to "stabilize". This occurs at a time when the temperature of the cloud and the temperature of the surrounding air are approximately equal. After stabilization, the mushroom continues to spread, initially because of the kinetic energy remaining after the rise, and later due to diffusion and Brownian motion™ of the. smaller particles. The time from detonation to the stabilization of an atomic cloud does not vary significantly with yield, but the height and _ rate of rise vary directly with yield. Figure 3 shows the predicted altitude of the cloud top and between what altitudes the cloud will lie. It will be noted that the cloud base tends to stabilize at the tropopause, a region in the atmosphere, generally at about 50,000 to 60,000 feet * Brownian motion: ‘The motion being attributed to the continuous bombardment of the particles by the molecules of the medium in which they are suspended. 17