projection of the path of a particle falling from some level in the stabilized cloud at a constant fall rate. The associated points on the surface represent the locations where the particles would land if they originated over ground zero at the given altitude. were drawn for 5,000-foot For the CASTLE Series, vectors (1,524-meter) increments of altitude, showing wind direction, and with lengths proportional to the time that 100-micron Particles would travel while falling through the layer. These wind-vector diagrams graphically illustrated the favorable or unfavorable wind patterns at the two shot atolls. Using a hodograph, direct time and distance meas- urements could be calculated (or "normalized") in terms of 100-micron par- ticles, and the behavior of any smaller sized particles could be estimated by taking multiples of the distances. Before BRAVO it was thought that particle sizes down to 70 microns were significant (which amounted to doubling the distances taken directly from the normalized hodograph). After BRAVO, however, particle sizes down to 50 microns were considered significant, and appropriate adjustments were made in the predicted fallout pattern. PARTICLE TRAJECTORY FORECASTS. Maps of the path that a particle at a constant altitude would take due to the winds were known as particle tra- jectory forecasts. These forecasts of trajectories from the shot site covered the period from H-hour to H+72 in 10,000-foot (3.05-km) increments ranging from 10,000 to 60,000 feet (3.05 to 18.30 km). The initial fore- cast was for H-hour and revised trajectories for the same 72-hour period were issued at H+6 and H+15. Radsafe personnel used the forecasts to as- sist in analysis of the long-range fallout aspects and to alert sampling units of probable locations and altitudes of areas with airborne radioactive particles. Data from past exercises used in fallout prediction consisted of particle-size measurements tudes) (which gave the rate of fall from various alti- and observations of downwind and crosswind variation of radiation intensity with distance from ground zero. cluded initial size of the radiation source 110 Additional considerations in(that is, the areal extent of