290 BULLETIN OF THE TORREY BOTANICAL CLUB [Vou. 91 temperature, the fireball expands and rises with great rapidity. The ‘‘primary’’ shock wave travels ahead of the expanding periphery of the fireball and produces a bowl-shaped cloud of dust when it strikes the ground. Some FIREBALL PRIMARY SHOCK WAVE REFLECTED SHOCK WAVE UPDRAFTS BEGIN IMMEDIATELY AFTER IMPACT OF PRIMARY SHOCK WAVE A. MACH FRONT, A WAVE OF INCREASED OVERPRESSURE TOROIDAL FIREBALL STEM WITH UPDRAFTS OF 200-300 MPH i y RY CLOUDS OF DUST PRODUCED BY MACH FRONT WIND OIRECTION Deere "CAP" FORMED AS FIREBALL COOLS AND IS DISSIPATED BY WIND “lS w= "LOCAL" FALLOUT OCCURS PRIMARILY pee IN DOWN-WIND DIRECTION “STEM” UPDRAFTS SOMEWHAT ABATED “ n” -~———— AFTERWINDS CONTINUE FOR SOME TIME Fig. 1. Stages in the development of a “mushroom cloud” following the detonation of a nuclear weapon a few hundred feet above ground. of this debris may be carried into the ‘‘stem’’ of the developing cloud by the high velocity updrafts which develop in the wakeof the rising fireball. While the fireball rises, it assumes a toroidal configuration which is similar to a rapidly-revolving, doughnut-shaped ring. Soil materials carried