surrounding the point of detonation, and no cloud, colum, or surge is formed. D. Effect of Meteorological Conditions on the Cloud. Four different types of action upon the atomic cloud can re- sult from the winds. When the wind structure is sufficiently large to embrace the whole cloud, gross movement occurs, i.e., the cloud as a whole moves in the direction in which the wind is blowing. When it is just, or very nearly, as large as the cloud, it helps to move it, but tends also to distort it as a whole. When the winds are appreciably smaller scale than the cloud, they cause the cloud to diffuse and lose its shape, because of eddy currents and dilution with clean air. Shear, the fourth action, tends to string out the cloud in ribbons by vertical deformation. The effect of wind shear upon a single radioactive par- ticle that is falling from its position at the time of stabilization is one of lateral translation. The amount of this lateral translation relative to the main axis of travel is a fimction, within any cloud segment of the wind speed of the segment and the length of time a particle spends falling through that segment. Some of these air movements tend to prolong the time that the large particles are in the air, increasing the horizontal translation by vertical wind motion during the free fall response to gravity. Particles that range in size from 100 to 5,000 microns are most likely to fall out within local fall-out contours from large yield detonations, although some smaller particles are also found. The par- ticle densities lie within a very narrow range of 2.5 to 2.8 gn/ec for Nevada tests, while at the Pacific Proving Ground they average 2.4 and range from 2.8 to 1.8 gm/ec. When the densities are essentially the same, the free fall rate is a function of particle size and atmospheric density. If constant particle density is assumed, the variation in the rate of fall from different cloud heights can be determined from Figure 5. Evidence indicates that from 50% to 90% of all radioactive particles in a fall-out area are within the size range of 50 to 1,000 microns in 22