in wind-sensitive contours which have only relative values. Actual values may be fitted to the contours as a result of one or more post-shot survey readings taken in the contaminated area. A prob- lem solution requires something more than an hour for personnel familiar with the method. U.S. Naval Radiological Defense Laboratory Method This is a wind and weather dependent system which essumes the bulk of the radioactive material originates in the lower portion of the mushroom, and utilizes an aerodynamic particle fall rate which varies considerably with the altitude of the particle. A problem solution requires several hours by personnel familiar with the method. The RAND Corporation Method The method gives wind and weather-sensitive contours, based on an assumed particle size distribution and the hypothesis that 90% of the fission product radioactivity falls out from the mishroom cloud and 10% from the stem. An aerodynamic rate of fall is used which is somewhat different. from that used by NRDL, but which also varies markedly with altitude. amount of hand calculation. A problem solution requires a great The method has also been programmed for machine solution. Technical Operations, Inc. Method The method utilizes an inverted cone cloud model, the NRDL assumption of particle size distribution, and fall rates which increase with increase in altitude. A problem solution requires somewhat more than an hour, utilizing trained personnel. There are two general categories into which the fall-out models can be placed. The first cetegory is that of a rough approximation which will furnish Military or Civil Defense officials with a knowledge of the approximate direction and area of the fall-out field from an estimate of the yield and winds. Such methods must be amenable to 53