4 THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD METEOROLOGY~-FALLOUT AND WEATHERING Since the air density decreases with altitude, the amountof radioactivity in a given volume of the cloud is much larger near the bottom than at the top. This appears to be borne out by actual fallout data with the one additonal fact that the peak activity seems to be located on somewhatlarger particles near the bottom of the mushroom cloud than near the top. In general, both im Nevada and the Pacific, the particle size with the greatest amount of mushroom radioactivity is between 100 and 150” in diameter with a specific gravity of about 2.5. APPLICATIONS Laboratories, the Rand Corporation, the University of California Radiation Laboratory, the Naval Radiological Defense Laboratory, and others. The result of the preceding analysis yields a model of a cloud from a specific explosion: A given yield, fission-fusion ratio, and type of burst. The best information, that from Nevada tests, is limited largely to comparatively lowyield weapons tested on towers. A scaling it is possible to provide estimates of the heights of origin and the particle size in a given part the upper left corner (from a paper by Dr. lines for the CASTLE BRAVOevent, together with the winds which carried the particles eastward. The remaining three figures are the authors’ estimate of what the same isolines would look like in different wind situations. In the lower left is a typical winter case of strong west winds, with an elongated fingerlike configuration. In the lower right is a case of light winds, changing from east to west. Note the marked difference in patterns. The upper right, a case of southerly low-level winds and moderately strong upper westerly winds, shows the stem fallout bulging northward somewhat in comparison with the lower left-hand case. of the radiation field. Ag indicated by particle size measurements, meteorological predictions do yield approximately the correct particle sizes; but along with the activity on the pre- of activity in the mushroom top and stem. The EXAMPLES that is, the radioactivity is mainly attached to It may be of interest to consider a typical prediction of fallout in the Nevada TestSite. In Figure 2 the predicted fallout in milliroent- The previous discussions and examples of fallout have considered only the effects of gravity and wind. It should be pointed out that if the airborne debris passes into an area of rain as the solid lines. The thin dashed lines ere the observed after-the-factfallout isolines in the on the quantitative effects of precipitation on of the fallout pattern, using a wind forecast made 2 hours before shot time and a Weather site is brought down byrain. Models proposed by various groups studying fallout have differed greatly in the proportions particle sizes in the stem are relatively larger, vide the details of each of the models of radio- uniformly distributed in the vertical, From first principles, one may argue that the thorough turbulentmixing in the mushroom will meke the amount of radioactivity per unit mass of air uniform throughout this part of the cloud. flavored by the Weather Bureau studies of Nevadatests. Not The bulk of the radioactivity in the fallout comes from the mushroom head. The ratio of such mushroom to stem material in Nevada bursts is roughly 3 to 1, but this distribution seems quite variable even on shots of similar yield, There probably is a smaller proportion of stem material in bursts in the megaton range. particles greater activity created by different organizations. Rather certain general results will be given, structure on the fallout pattern is shown. Gordon Dunning) is a set of idealized dosage For one thing, it is comparatively which is necessary in estimating the cumula~ tive dose from a dose-rate measurement. Also, if there is fractionation in the nuclear. cloud as a function of altitude or particle size, then formula is required to refer the dosages to other yields and heights of burst, but such scaling velationships are not yet well understood. It does not appear to be appropriate to pro- Nevada, missed in the previous static wind cases, is better accountedfor in this figure. In Figure 5 the pronouncedeffect of the wind posium. easy to estimate the time of arrival of fallout, practicallyall groups engaged in this procedure: the shot time winds are used. A comparatively smal) decrease in wind velocities has made the fallout pattern shorter and wider than the H-2 forecast. Finally, as shown in Figure 4, a more refined treatment of the wind has been attempted. The time and space changes of the wind along the pathsofthefalling particles have been incorporated. It is evident that the eastwardturning of the fallout pattern in northern only the bearing of the fallout pattern, but also the shape is controlled by winds. Shown in features which may be of interest in this sym- The Weather Bureau, the Los Alamos Scientific Bureau model of radioactivity. Such forecasts are used by the test management in making the decision whether or not to fire. On Figure 3, intensity field is predicted, there are other Aside from demonstrating how afallout FraurE L—WHypothetical Fallout Plot. 5 than about 200 microns. Further, the lower down in the stem, the larger the particles appear to be. The activity in the mushroom is also non- dicted particle sizes, there is a disturbingly large fraction of activity on particles too small (even less than 5 microns) to have a significant settling velocity. gens per hour 12 hours #fter the burst is shown same units. This case shows the verification SCAVENGING BY PRECIPITATION or snow, a very different radiation pattern on the ground mayresult. There is no good data close-in fallout, but it has been observed that most of the radioactivity remote from the test