of such large particles (70 to 150 microns), it is possible to prepare a simple wind vector plot indicating where the fall-out will touch the ground from a given point in the atomic clouwi. These wind plots have been used previously under various names and have been described in great detail (3, 4, 5, 6, 7), therefore no attempt will be made to describe the method in this report. However, such vector wind plots have been used extensively by this writer to obtain a lot of indirect _dnformation, There is some indication that soil particle size decreases with altitude in the cloud. It should be clearly understood that the particle sizes indicated above refer to the median soil particle diam eter, and that the soil particle size spectrum is wide, The fall-out at a given spot may have comefrom different levels of the cloud, thus further increasing the spread of the size spectrum, The density of particles at NPG average around 2.5 gn/cm’, but certainly not all the particles would have the same density oor are they all spherical in shape and this also increases the particle size distribution. Strangely enough, during the domestic test operations it was observed that many particles in the size range of only several microns in diameter fell cut within a few hours after bomb burst. According to Stokes’ Law (even when corrected for the Cunningham slip factor and for the variatien of air viscosity with temperature) it would take a 5 micron particle several months to reach the ground from 40,000 “. The explanation is toe be found in the fact that a large: quantity of soil is sucked up into the cloud and as this soil subsequently falls back to the ground, it entrains and traps a lot of air and a lot of small sized primary fission leben te ntl UNCLASSIFIED 13 et