376 MACHTA WIND = GROWTH DUE TO NORMAL " HORIZONTAL MIXING TOP VIEW Bz GROWTH DUE TO WIND SHEAR AND VERTICAL MIXING SIDE VIEW Fig. 6—Schematic representation of cloud grouth due to diffusion, wind shear, and vertical mixing. 24°N, whereas at higher altitudes, e.g., 24,000 ft, the trajectory crossed the flight line at about 21.5°N. In this case, the width of the peak con- centration samples covers 4 to 5 degrees of latitude. This is a case of moderate directional shear. STRATOSPHERIC FALLOUT The greater interest in predictions of stratospheric than of tropo- spheric fallout arises from Several considerations: so far, only stratoSpheric debris from weapon tests is still available to fall out; the largest fraction by far of long-lived fission products has been injecteca into the stratosphere by nuclear explosions; and there is at present a livelier scientific interest in stratospheric than in tropospheric trans- port and diffusion, With two important exceptions, short-lived radio- activity decays in the stratosphere before reaching the ground; thus only long-lived isotopes should be considered. The exceptions are the Scavenging of radioactivity by thundershowers penetrating into the stratosphere’ and the quick exit from the stratosphere postulated by Danielsen,® Reiter,‘ and others. It should also be noted that timeof fallout after a test series has been the prime means of distinguishing between tropospheric and