by the average reading on a rediac instrument. One such example would be a ditch in an area with a surface wind of sufficient intensity to blow particles into such depressions after the initial deposition of fall-out. Troops using these depressions for protection might be ex- posed to more external ganma radiation than a meter in the open would indicate. ‘The beta activity in such depressions would also be rela- tively higher. If the fall-out occurred on a sea area, for example, while troops are being transported on ships, the ships would be the primary areas of high levels of radiation. Fall-out material falling into the water is somewhat soluble and in addition the majority of the particles tend to settle with sufficient rapidity to decrease the radiation intensity to a non-hazardous level within a short time, whereas the same amount of activity deposited on a land surface could be very hazardous. A discussion of the biological effects of radiation from a fallout area will be given in the section on hazard evaluation. Suffice it to say here that knowledge of isodose contours aids materially in the evaluation. Further discussion of the world-wide fall-out situation will be given in other sections of this report. The height of burst above the surface of the ground has a marked effect upon the percentage of expected fall-out from a true surface burst. ered. In previous examples, only contact ground bursts were considAs a rough rule of thumb, the percentage of total fall-out available that is deposited within local contours is about the seme as the percentage of fireball volume subtended by the earth. Thus, for a contact surface burst, roughly 50% of the available fission product activity is deposited within localized contours. Figure 11 indicates the relative degree of contamination to be expected from near-surface bursts as compared to contact surface bursts. The general shape of the fall-out pattern is seen to be approximately the same for a burst very close to the surface as for a surface burst, but the level of activity is different by the percentage indicated. 46 Table 2 has been